There are a lot of myths, speculation, and just plain BS about the 9mm AR. Between obsolete “9mm hammers”, un-ramped bolts, too-light “9mm buffers”, and all the horrible advice, it’s amazing anyone can get them to work right. The first 9mm AR that I assembled had many issues, requiring months of research and experimentation. It took me almost a year to get it running 100% and now it’s running perfectly.
It has taken a lot of work to filter through all the worthless and confused heresay from ancient Arfcom posts and “Fudd lore” internet articles. Here is the useful, practial, and, as best as I can tell, accurate information, finally all organized and posted in one place. It’s a lot of information, but if you’re really interested in what makes a 9mm AR tick, and what can help it run like a Swiss watch, this is for you.
This information is intended solely for entertainment purposes only. Do not perform any action based on any of the following information. The accuracy of this information is not guaranteed or warranted.
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AR-pattern 9mm simple blowback basics:
- The original AR-pattern 9mm simple blowback operated SMG/carbines were developed by Colt in the 1980’s. Owners report they use a heavy unramped 15.9 oz. bolt, a carbine length (3.25″) 5.6 oz. sliding-weight buffer (bolt + buffer reciprocating weight of 21.5 oz.), an aluminum buffer spacer (0.75″, Colt part number SP400211) installed in the back end of a standard carbine recoil spring, and a carbine buffer tube.
- The Colt-style magazine is based on the Uzi mag, with the mag catch moved and last round bolt hold open (LRBHO) capability added.
- Solid steel buffers were used at first, but they switched to sliding weight buffers after reports of out-of-battery discharges from bolt bounce.
- The 9mm bolt is about 0.75″ shorter than the 5.56mm bolt and carrier. The 9mm bolt is essentially just a 5.56mm bolt carrier with no protruding bolt, the bolt face machined into the flat front of the carrier, and a weight pinned in the hollow rear.
- A buffer spacer added to the rear of the recoil spring limits the rearward travel of the shorter bolt and helps prevent bolt catch damage. It also helps prevent fired cases from getting jammed in the fire control group pocket of the lower. Extended length buffers (about 4.0” long) are available that are designed to eliminate the need for a buffer spacer.
- 22 oz. – 24 oz. is a bolt + buffer weight range that seems to work well with the vast majority of configurations and ammunition types.
- Bolts available on the consumer market generally seem to weigh 12oz. – 15.5oz.
- 9mm buffers can weigh up to 12 oz. The old 5.6oz. “9mm buffer” should be considered too light to use with modern 9mm bolts.
- Aluminum cased ammo is generally not recommended in simple blowback systems.
- Simple blowback systems get dirty quickly and may be rough on parts. Keeping everything clean, lubricated, and carefully inspected should help prevent a number of potential problems.
- The most common problems that come up with 9mm blowbacks are ejection issues and chambering/feeding issues (see here for more info.)
- A lot more information about Colt’s 9mm can be found on the SmallArmsSolutions channel on YouTube. I wish I had known about these videos long ago. He covers a lot of things I had to learn on my own.
What is “simple” blowback (a.k.a. “direct”, “plain”, “straight” blowback)?
- In a simple blowback firearm operating system, there are no locking bolt lugs, or any other mechanical locking or delay mechanisms, to keep the breech closed during firing. When a cartridge is fired, high pressure gasses push the bullet in one direction down the barrel, and the (now empty) cartridge case is pushed in the opposite direction out of the chamber. Movement of the thin-walled cartridge case has to be minimized until the bullet has left the other end of the barrel and the gas pressure can release. Otherwise, the thin walls of the cartridge case could burst from pressure as it is pushed out of the chamber, blowing cartridge case fragments and high pressure gasses in all directions and potentially causing injury.
- Slowing the extraction of the cartridge case is achieved by the heavy buffer and bolt behind the cartridge. The bolt and buffer want to remain at rest, resist the rearward push, and move slower than the relatively lightweight bullet heading in the opposite direction. In a properly weighted and balanced system, the bullet and the high pressure gasses fly out of the muzzle end of the barrel well before the fired cartridge case moves far enough to rupture.
Build vs. buy
- Many people are attracted to the idea of building their own 9mm simple blowback AR because of the cost savings over purchasing a completed firearm from a large manufacturer. Some people just want to build a completely custom 9mm AR or enjoy the challenge. Many people have successfully completed builds without any (or just minor) issues.
- Some people have had serious issues with their builds which resulted in the expenditure of a lot more time and money than they had originally planned. This may include replacing practically all of the parts until a combination is found that works properly; spending a lot of money on tools, ammo, and range trips; and spending months getting it all sorted out. It is possible to spend as much or more than the cost of a top quality complete 9mm AR.
- Purchasing a complete 9mm AR from a manufacturer with a good reputation, good customer service, and a solid warranty may have significant advantages. They should have already gone through all the troubleshooting, testing, ammo, money, and frustration to make sure their particular combination of parts works correctly. If it does have problems, it can usually be sent back for warranty repair and it will be their problem to figure out what is wrong and to make it work.
- Before you start a build, please read this to properly set your expectations. Slapping together an AR9 from parts is easy. Building one that actually works is a different story. It’s not difficult, but requires learning and understanding some of the intricacies of the system.
Parts choices make a difference
The following parts individually seem to solve various problems, or appear to cause fewer problems, with 9mm AR platform builds.
- A barrel with a generous feed cone/ramp (go here for a barrel list). This is the most critical part of getting a 9mm AR to feed properly. More information here: AR9 barrel install/replacement
- A ramped, proper weight, 9mm bolt from one of the larger reputable manufacturers.
- A carbine-length buffer tube.
- A heavy extended-length (4″, or A5-length) buffer with sliding weights inside. For help selecting a buffer: What 9mm buffer do I need??? (and spring)
- A total reciprocating mass (bolt + buffer weight) of around 22 oz. or more (usually requires a 7.5 oz. buffer minimum)
- A standard strength 5.56 carbine recoil spring.
- A standard “mil-spec”-type semi-auto trigger set with “DPMS-style hammer” (most common hammer currently made).
- A standard strength hammer spring and trigger spring.
- A properly and completely machined, in-spec., aluminum receiver from a reputable company.
- OEM or very high quality magazines with a good reputation.
Parts to avoid
Be aware that just because a part is advertised as “for 9mm” or “PCC” doesn’t mean it’s true, or that it’s a good idea to use it. The following are a few of the parts that generally cause more problems in a 9mm blowback system and should either be avoided or never used at all.
- “9mm specific” hammer (notch cut in middle of face).
- Do not use. Obsolete, and was never actually needed for 9mm. If paired with a modern ramped bolt it may jam or fail to reset.
- “AR-15 specific” hammer (notch, or “step”, cut at front top of face).
- Do not use. Obsolete, may cause bolt to jam in receiver.
- An unramped bolt.
- Do not use. Obsolete. Causes excessive hammer pin stress/breakage.
- Bolt missing bottom feed lug on bolt face.
- Do not use. Potential for out-of-battery detonation of the cartridge if the bolt stops short of fully chambering the cartridge. See here for pictures and details.
- A light hammer spring.
- Do not use. May cause doubling or runaway.
- A .308 recoil spring (includes Wolf XP and Sprinco Red).
- Do not use. Grossly overpowered for 9mm blowback and may cause bullet setback/overpressure, trigger reset failures, nosedives, stovepipe jams, and conceal feed issues.
- An extra-power recoil spring (e.g. Sprinco Blue, Sprinco White-Hot, etc.)
- Avoid, unless a higher bolt cyclic rate is specifically required to resolve hammer-follow issues using binary triggers, full auto triggers, or exceptionally fast competition triggers.
- Captured spring systems (including “silent” spring systems)
- Do not use. Require removal of the internal bolt weight, resulting in an undermass system. Insufficient reciprocating mass and oversprung.
- A polymer receiver.
- Do not use. Most are not designed for the violent blowback forces. May crack/break.
- Exception: KE Arms poly lower is reported to work with 9mm PCC builds with 5.4 oz. buffer minimum.
- An 80% “finish it yourself” receiver.
- Do not use. 80% lowers add an additional and unnecessary set of variables to the equation, and there appears to be a significantly greater chance of having out-of-spec issues and operational failures.
- A solid buffer (no sliding weight inside).
- Do not use. Causes bolt bounce. May result in out-of-battery case ruptures during controlled pairs or rapid fire.
- A carbine-length buffer.
- Avoid, unless paired with a buffer spacer. Causes bolt rearward overtravel. May cause broken bolt catches. Allows cartridge cases to fall into and jam the trigger group.
- Aftermarket triggers designed for 5.56mm AR-15.
- Avoid, unless specifically approved by the maker for PCC, or has a proven history in PCC. There is no agreed-upon standard for 9mm bolt machining like 5.56mm bolts. Some triggers may work, some won’t.
- An A5 buffer tube.
- Avoid, unless paired with an extended (4″, or A5) length buffer AND a buffer spacer. Using an A5 buffer tube will cause bolt rearward overtravel, even with an extended (4″) length buffer. A buffer spacer can be combined with a 4″ buffer to make up for the extra space in the tube and allow for proper bolt travel.
- Micro buffer tubes shorter than 7″ internal length.
- Do not use. These use shortened tiny solid buffers with insufficient mass for blowback and no deadblow effect to prevent bolt bounce. All are severely oversprung.
- Cheap magazines
Magwell adapters vs. dedicated PCC lowers
- Magwell adapters are an option for people with MG or SBR AR lowers, or anyone who would like to use their current AR lower for 9mm. Adapters may require some fitting and adjustment to work properly. Some include last round bolt hold open (LRBHO) capability. Some adapters cost as much or more than dedicated lowers.
- Magwell adapters are available that use mags for Colt (modified Uzi), Glock, S&W M&P, Beretta 92, Sig P320, and others.
- Some magwell adapters use a separate mag catch/release button, usually located at the bottom of the magwell adapter. The original mag catch in the lower holds the adapter in place. To prevent ejecting the magwell adapter by accident, some users install a CA/NY compliance “bullet button” or internal magazine locking system in the lower receiver.
- Dedicated lowers often come from the factory with the mag catch and ejector installed. Some have LRBHO capability, some do not.
- There are lowers available for Colt, Glock, MP5, Scorpion Evo, and other mags.
- Some bolts are not compatible with some magwell adapters because of the bottom contour of the bolt. Bolts that look like this appear to be more compatible and tend to have sufficient clearance:
- See also: Fix for a slipping Colt mag adapter
Extra-power or .308 recoil springs vs. standard carbine recoil spring
- The idea that a 9mm AR needs a .308 recoil spring is a myth.
- 9mm blowback does not need an XP, Red, hot, stiff, firm, etc. recoil spring to operate properly.
- Colt’s 9mm carbines, based on the 5.56mm receiver, were designed to use a standard 5.56mm carbine recoil spring.
- .308 springs are intended to be used in AR-10 (.308) receivers which are substantially larger and stronger than 5.56mm/9mm receivers, and can withstand the beating of the stronger spring over time.
- A .308 spring may conceal feeding issues caused by poor feed cone design.
- An extra-power or .308 recoil spring should never be used as a substitute for having a proper weight buffer and bolt, or to attempt to “hold the bolt closed longer”.
- An extra-power or .308 spring may slightly slow the rearward acceleration of the bolt but does not significantly help keep the breech closed after firing.
- An extra-power recoil spring forces the bolt forward faster and harder than a regular spring when returning the bolt to battery. This reduces bolt cycle time, increases the force used to strip cartridges out of the magazine, causes more muzzle dip, and puts extra stress and wear on bolt catch parts and trigger group parts, possibly causing breakage.
- Increasing the forward bolt speed may not give brass enough time to eject, trapping brass in the ejection port (stovepipe jam).
- Some users have reported that .308 springs forced the rounds to feed too quickly, causing the cartridge to ram into the bottom of the breech face (nosedives).
- .308 rifle recoil springs may force a cartridge to chamber after slamming the cartridge into the breechface, forcing the bullet backward and deeper into the case (“bullet setback”). This reduces the space in the cartridge case for the burning powder to expand, and can result in an overpressure situation when the cartridge is fired, potentially causing damage and injury.
- Bolt dwell time may be too short for reliable trigger reset when using a .308 recoil spring.
- Very stiff springs may reverberate less when cycling, reducing spring noise during firing.
- Very stiff springs may reduce bolt bounce from cheap solid buffers, allowing manufacturers to avoid the expense of including a proper (and more expensive) deadblow-type buffer with their guns.
Anti-rotation pins or stainless steel pins
- In early 9mm simple blowback ARs, the unramped bolts forced the hammer to reset very hard and fast. Combined with the violent action of the blowback system and the high cyclic rate of the SMG configuration, unramped bolts broke hammer pins frequently. Colt found that replacing the standard pins with stronger stainless steel pins increased the time before breakage. These stronger pins have a reputation for damaging the receiver pin holes. Using a modern ramped bolt resolves this problem entirely.
- Some people add anti-rotation pins to their AR to prevent the receiver pin holes from being damaged, which may be more important to someone with a registered SBR or MG lower. Anti-rotation pins that are connected to each other on the outside of the receiver with external struts may help distribute the force through both pins equally, reducing the chance of damage to the pins or the receiver holes.
Barrel length
- 9mm tends to increase in velocity as barrel length increases, with the largest gains generally seen as length increases up to 8″.
- Beyond 8″ of barrel travel, the average % velocity increase per inch is significantly less, and often inconsistent, depending on cartridge load.
- See Ballistics by the inch – 9mm results for details.
- Longer barrel lengths may cause some subsonic loads to become supersonic, affecting suppressor performance.
- Longer barrel lengths may cause poor terminal performance from hollow point ammunition designed for typical pistol barrel lengths.
- Shorter barrel lengths may result in the appearance of yellow flakes of partially burnt powder in the action, depending on the ammunition used and the total reciprocating mass.
- More information here: AR9 barrel install/replacement
Handguard length vs. barrel length
- 9mm barrel length is measured by MOST manufacturers as the actual barrel length, chamber face to muzzle, since there is no barrel extension like with 5.56.
- Because there’s no barrel extension screwed onto the rear of the barrel, the barrel sits about 0.501″ further back. So the muzzle is also about 0.501″ further back compared to a 5.56 barrel. If you are familiar with 5.56 barrel/handguard ratios, get a 9mm barrel 0.5″ longer than a 5.56 barrel for the same handguard.
- The barrel nut threads sit about 0.44″ out from the front of the upper receiver. The extension sits about 0.99″ inside the receiver, so the barrel end sits about 0.55″ behind the rear of a free-float handguard (this applies to both 5.56 and 9mm).
- The length of the threads on the muzzle varies by manufacturer, usually between 0.4″ and 0.625″ long. This affects how far back the muzzle thread shoulder sits behind the muzzle.
For 9mm, generally, if:
- Handguard length = barrel length (e.g. 8″ handguard, 8″ barrel): Muzzle is about 0.55″ under the handguard. Thread shoulder is about (thread length + 0.55)” under the handguard.
- Handguard is 1/2″ shorter than barrel (e.g. 7.5″ handguard, 8″ barrel): Muzzle is about 0.05″ under the handguard. Thread shoulder is about (thread length + 0.05)” under the handguard.
- Handguard is 1″ shorter than barrel (e.g. 7″ handguard, 8″ barrel): Muzzle is about 0.45″ outside the handguard. Thread shoulder is about (thread length – 0.45)” under the handguard, if positive. If this is a negative number, it’s outside the handguard.
- Handguard is 1 1/2″ shorter than barrel (e.g. 6.5″ handguard, 8″ barrel): Muzzle is about 0.95″ outside the handguard. Thread shoulder is about (0.95″ – thread length) outside the handguard.
The Law Tactical folding mechanism
- 9mm bolts are patterned after the 5.56mm bolt carrier, but they need to weigh significantly more to allow for safe operation of the simple blowback system. Additional weight is usually added in one of two ways: an additional weight plug is fastened inside the back of the hollow bolt, or the bolt is split into two parts with a removable solid heavy cylinder at the rear end.
- To use a Law Tactical AR folding adapter, the bolt needs to be the type with an internal weight plug fastened inside the hollow rear of a full-length bolt.
- For a Law folder, the inner diameter of the hole in the hollow back end of the bolt needs to be around .625″, and the internal weight needs to be removable (they are usually held with a horizontal roll pin). This is because the Law folder uses a bolt extension that plugs into the back of a standard length hollow bolt carrier. I can confirm that my Brownells Hybrid 9mm bolt (NOT the Glock-only one) and Kaw Valley Precision 9mm bolt work with the Law folders.
- For bolts with a .531″ inner diameter, Freedom Ordinance makes an adapter but it’s polymer, so a very heavy buffer would be needed to make up the missing bolt mass.
- The Law bolt extension weighs about 2.0 oz., so it will only make up some of the weight lost (usually 3.5 oz.) from removing the internal bolt weight.
- KAK makes a custom heavy extension plug (3.9 oz.) for the Law folder that is a drop-in replacement for PCC applications to both provide the necessary extension and make up the lost weight.
- Some people have successfully trimmed and reinstalled the internal bolt weight so that both the shortened weight and the Law extension fit properly. Others have used a heavier buffer to make up the lost weight.
- The Law Tactical adapter adds 1.3″ to the “length of pull” (LOP).
Sylvan Arms folding mechanism:
- Some of the information above may apply to the Sylvan folder mechanism. They are similar, but none of the Law and Sylvan parts are interchangable. They are similar in that both require removing the internal bolt weight, and both use a similar method to extend the bolt through the hinge, but the bolt extension plugs and LOP are different.
- The Sylvan folding adapter can use either the hollow type of bolt or the split-type, with some exceptions. Sylvan makes an adapter (Gen 3 folder only) for one kind of split-type bolt that will allow them to accept their bolt extension. Used for bolts from Palmetto State Armory, Diamondback Firearms, Anderson Arms, New Frontier, and Aero Precision.
- Sylvan also makes a smaller diameter extension (.531) for bolts with a smaller inner diameter. Gen 3 folder only. This is NOT for use with the Law Tactical folder.
- Sylvan recently (2022) came out with a Gen 4 folder, but the bolt adapters mentioned above are not available as of early 2023.
Franklin Armory BFSIII in a 9mm AR
- Getting the Franklin Armory BFSIII to work properly in a 9mm AR can require some tuning. “Hammer follow” is the most frequently reported issue.
- “Hammer follow” appears to be the result of either “outrunning the trigger” (hammer releases as the bolt is still moving forward and comes to rest behind the bolt without firing a cartridge), or “bolt bounce” (hammer falls forward as the bolt “bounces” backwards, preventing the hammer from striking the firing pin and causing the hammer to come to rest behind the bolt). In both situations the hammer may be resting against the rear of the firing pin with an unfired cartridge in the chamber. A light primer strike may be evident.
- Buffers with sliding internal weights help prevent “bolt bounce”.
- Getting the bolt into battery faster should help prevent “outrunning the trigger”. This may be possible by reducing overall bolt travel or increasing recoil spring pressure. The Franklin “medium” recoil spring (silver w/black end) that comes with the kit is reported to help and is probably a good starting point for most people.
- Keeping the bolt+buffer mass around 21-22 oz. is reported to help. Heavier masses will slow the bolt cycling time, increasing the chance for hammer follow.
- Limiting use of the trigger to quick binary “double taps” (pull-release-pause, pull-release-pause…) seems to prevent hammer follow. “Racing” the trigger through multiple binary cycles is more likely to cause hammer follow.
- All safety instructions and testing protocols provided by the manufacturer must be followed.
Putting a 9mm upper on a 5.56mm AR
- Some people have installed a 9mm magwell adapter of some type into a 5.56mm AR, swapped the 5.56mm upper for a 9mm upper, and started using it without any immediately apparent negative consequences.
- The typical 5.56mm buffer is much lighter than a 9mm-specific buffer. A light buffer may result in increased wear and stress on internal parts and could be dangerous to operate.
- Swapping the 5.56mm buffer for a 9mm-specific extended-length (about 4.0”) buffer should be considered a necessary part of the swap. It helps reduce perceived recoil, reduces stress and wear on the system, limits bolt over-travel, reduces the chances of damaging the bolt catch during last round bolt hold open, and help prevent an empty case from falling into the trigger group pocket of the lower. For help selecting a buffer: What 9mm buffer do I need??? (and spring)
- Brownells produced a good video explaining what needs to change to use 9mm with a 5.56mm AR.
Buffer weight
For help selecting a buffer: What 9mm buffer do I need??? (and spring)
- Colt originally included a solid 5.5oz. carbine length buffer for use with their 16oz bolt.
- Solid buffers are prone to bolt bounce, which can cause hammer-follow and out-of-battery discharges.
- Many current manufacturers still produce solid buffers and try to get away with including these low-quality buffers in their builds due to their dirt cheap production cost, often pairing them with a grossly overpowered recoil spring.
- Because 9mm bolts are shorter than 5.56mm bolts, carbine-length buffers require an additional spacer (see picture above) to prevent rearward bolt overtravel.
- 22-24oz. bolt+buffer weight (reciprocating mass) is compatible with most 9mm AR blowback configurations and factory ammunition types. For most builds, this means using at least a 6.5 oz. buffer, or greater, depending on bolt weight.
- Less than 22oz. can cause excessive bolt velocity, accelerated parts wear/damage, and increased perceived recoil by decreasing cycle time.
- Heavier reciprocating masses decrease bolt velocity, decrease wear/damage on parts, and reduce perceived (felt) recoil by increasing cycle time.
- 9mm blowback has been shown to consistently cycle over 32oz. of total reciprocating mass with an extremely short 2.5″ barrel and bulk 115gr. ammo. The Overmass Project: World’s heaviest 9mm blowback action? (with videos). It is essentially impossible to put too much bolt/buffer weight in a typical 9mm AR build.
The following are examples of buffers that I would avoid because they are either too light (under 6.5oz.) to reach 22oz reciprocating mass with most current-production bolts, are solid and do not have deadblow weights inside to mitigate bolt bounce, or are a short proprietary length.
Buffers I would AVOID in a 9mm build:
3.0 oz. – 5.56mm standard carbine (too light)
3.7 oz. – 5.56mm H1 carbine (too light)
4.6 oz. – 5.56mm H2 carbine (too light)
5.2 oz. – 5.56mm rifle-length buffer (too light)
5.5 oz. – Generic “9mm” carbine (solid, too light)
5.5 oz. – 5.56mm H3 carbine (too light)
5.5 oz. – Blitzkrieg elastomeric (too light)
6.0 oz. – Blitzkrieg hydraulic (too light)
6.0 oz. – B&T Hydraulic (too light)
6.1 oz. – Vltor A5H3 (too light)
6.4 oz. – Vltor A5SH0 (too light)
6.5 oz. – Foxtrot Mike FM-9 (solid, proprietary length)
7.5 oz. – Kaw Valley Precision (solid)
7.5 oz. – Foxtrot Mike FM-9 (proprietary length)
7.7 oz. – Spikes tactical ST-9X (solid)
7.7 oz. – Aero Precision EPC (solid)
8.0 oz. – KAK Industries (solid)
8.5 oz. – Slash’s 9mm-Q Buffer (solid)
Commercial buffer spacers
- The 9mm bolt is about 0.75″ shorter than the 5.56mm bolt and carrier. When using a carbine-length (3.25″) buffer, a buffer spacer installed at the back of the recoil spring limits the rearward travel of the shorter 9mm bolt in a carbine buffer tube. This helps prevent bolt catch damage and also prevents fired cases from getting jammed in the fire control pocket of the receiver.
- An extended-length buffer (~4″) generally eliminates the need for a buffer spacer.
- Commercially-made buffer spacers (~0.75″) are made by Spikes Tactical, New Frontier Armory, Ergo, Wolfpack Armory, and others.
- Another option for a 3/4″ (0.75″) spacer is an aluminum standoff for signage. It even has a hole in the center for airflow.
- Buffer spacers are inserted into the opposite end of the recoil spring from the buffer, and inserted first into the buffer tube when reinstalling the spring.
U.S. quarters as a buffer spacer?
- It’s fairly common to find the suggestion to drop several U.S. quarters into the empty buffer tube behind the recoil spring to act as a cheap and customizable-length buffer spacer when using a carbine length (3.25”) 9mm buffer.
- A stack of U.S. quarters is usually cheaper than purchasing most commercial buffer spacers or extended buffers.
- They allow a user to customize the distance between the bolt catch and the bolt face at the fully rearward bolt position.
- If the space between the bolt and the bolt catch is reduced too much by the over-application of quarters, the bolt may override the catch before it has time to pop up into position, defeating the last round bolt hold open feature.
- Since quarters are nearly the same diameter as the inside of the buffer tube, the recoil spring sits on top of them, compressing the spring and causing it to apply more force when cycling.
- U.S. quarters may deform (become bowl shaped) over time from the repeated impact of the buffer.
9mm hammers and “ramped” bolts
Typical AR hammer. Also called “DPMS style”. Works with almost every 9mm bolt.
- Avoid “9mm-specific” hammers with a notch in the middle of the hammer face. These hammers are still made, but are obsolete and frequently cause jams or other issues. They were originally paired with “unramped” bolts (see below) in Colt “civilian” semi-auto-only 16″ 9mm ARs. 9mm-specific hammers are not beneficial or required for a 9mm AR. These hammers can fail to reset when paired with a modern ramped bolt.
- 9mm bolts in both Colt’s SMGs and semi-only guns were “unramped”, meaning they were machined with almost no bevel under the rear of the firing pin where it resets the hammer. Semi-only guns were paired with a “9mm-specific” hammer with a large notch in the middle of the face that was designed to jam against the unramped area if someone tampered with the trigger system in an attempt to make it “full auto”.
- Unramped bolts were tested by users and found to work with most hammers. 9mm-specific notched hammers were unnecessary.
- Unramped 9mm bolts reset the hammer with a lot of force and put a lot of stress on the hammer pin, causing them to break.
- “Ramping” the 9mm bolt cuts a larger/deeper bevel (ramp) under the rear of the firing pin, giving the bolt more leverage for resetting the hammer, increasing the hammer reset time, reducing stress on the hammer pin, and rendering the “9mm specific” notched hammer design completely obsolete.
- Most current production bolts come “ramped” from the manufacturer. Un-ramped bolts can be made “ramped” by a number of machine shops.
- Avoid hammers with a 90 deg. notch cut in the very top front of the hammer face. Sometimes called “AR-15 Rock River” or “AR-15 specific” hammers, they were designed to catch the firing pin collar in early semi-auto AR-15 “unshrouded firing pin” bolt carriers in case of disconnector failure. These hammers are obsolete and can jam the carrier in the receiver, and can fail to reset when paired with a modern ramped bolt.
- More specific info here: What hammer do I need for my 9mm AR? (hammer/bolt compatibility).
Aluminum cased ammo
- Aluminum cased ammo appears to be more likely to cause problems in simple blowback operated firearms and the general consensus is to avoid it.
- Some people have used it successfully, some have had poor results.
- Torn rims, torn cases, ruptured cases, and extraction failures may be more likely in a simple blowback 9mm AR when using aluminum cased ammo.
- Some people have reported it worked OK in their build with an 8 oz.+ buffer, but had case failures/ruptures with lighter buffers.
- Although a relatively heavy reciprocating mass (bolt+buffer weight) may allow aluminum cased ammo to function safely, the minimum limit has not been defined.
- More info can be found here: Aluminum case ammo – Can I use it in my blowback 9mm?
Common problems
- Empty case ejection issues
- Bolt bounce
- Hammer follow
- Out-of-battery issues
- General feeding issues
- Bolt rearward over-travel
- Broken bolt catches
- Bulged or ruptured fired cartridge cases
All of the common problems with the 9mm AR platform are symptoms of the design concessions made by the engineers at Colt when converting their rifle cartridge receivers to operate with short 9mm pistol ammo. The conversion was designed to use as few new parts as possible to keep it cost effective. This resulted in many compromises and retrofitted parts.
All of these problems can usually be resolved by following the steps found here: 4-step guide to make (almost) any AR9 run 100%
For more detailed troubleshooting, please check the 9mm AR Troubleshooting Guide.
Zero distance
User “LordWesquire” on Reddit figured out the optimal sight zero distance for 9mm with a sight-over-bore of 2.5″ (common for red-dots) and 9mm 115gr. traveling at 1200fps. It’s about 20.5 yards or 85 yards (both give the same ballistic results). This gives the following point of impact results:
- 5 yards -1.78″
- 10 yards -1.14″
- 15 yards -0.56″
- 20.5 yrd ±0.00″
- 25 yards +0.39″
- 50 yards +1.57″
- 75 yards +0.86″
- 85 yards ±0.00″
- 100 yards -1.90″
This is all dependent on the sight-over-bore height of the sights and the particular load, but it seems like a good general rule that 20-21 yards is a good setting that keeps the bullets within about 2″ of the point of aim from 5-100 yards.
Blowback9.com 