Could this be a drop-in CMMG RDB killer? TL;DR: Almost, but not quite.
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Maxim Defense rather quietly rolled out a new product that could have a big impact on the 9mm AR industry. A drop-in, self-contained, fully captured, roller-delayed buffer/spring system that replaces the standard buffer, buffer spring, and bolt weight in a carbine 9mm AR. They also make a version for their shorter Maxim PDW buffer tubes.
https://maximdefense.com/product/244978/
It just so happens that I have an MP5. Let’s see out how well this system works and how it compares to both an MP5 and my Gentle Recoil Buffer System.
Operation:
With an MP5, the bolt face is locked into the trunion by the rollers. The inertia of the cartridge pushing on the bolt face has to overcome the mechanical advantage of the “locking piece” holding the rollers in place. Eventually the force is too great for the locking piece and it moves out of the way, allowing the rollers to unlock.
With the Maxim setup, the rollers (ball bearings) are behind the bolt inside a 2-piece self-contained buffer. The front of the buffer is locked to the guide rod by the “rollers” that rest in a groove in the rod. The rear portion of the buffer has a collar that surrounds and locks the bearings in place. The collar has an inclined cut so it’s shaped like a funnel. The angled surface acts much like the angled locking piece of the MP5. It’s basically a self-contained, inside-out version of the MP5 system.
Upon firing, the blowback forces push on the front portion of the buffer, but it needs to move the rear portion out of the way first to unlock the bearings. As the pressure increases, the bearings try to push outward and start moving the collar backwards. This eventually moves the collar out of the way, releases the bearings, unlocking the front portion of the buffer from the guide rod.
It’s this momentary mechanical delay which should reduce felt recoil.
9mm bolt mass and the internal pinned-in weight:
Because the Maxim captured system uses a guide rod, the AR bolt needs to be hollow at the rear. This means removing the bolt weight pinned in the rear of almost every 9mm blowback bolt is necessary. That weight was put in the bolt for a reason and is normally required for safe operation with simple blowback. The mechanical delay of the Maxim system converts the operating system to delayed blowback, which makes the extra weight unnecessary.
This also means that the Maxim system is not compatible with 2-piece bolts with a solid rear section, such as the Aero Precision 9mm bolt.
A typical AR 9mm bolt without the rear weight is about 12oz. There are no obvious provisions to prevent bolt bounce with the Maxim system.
My MP5 bolt/carrier assembly (minus guide rod and spring) weighs 9.6oz. This includes 1.15oz of tungsten granules inside the bolt carrier to reduce bolt bounce.
Folding buffer tube systems:
The Maxim system is not directly compatible with buffer tube folders, like the Law folder, because the bolt extension is solid. We would need a hollow bolt extension for the guide rod to travel through. Luckily, there is one! I found one and ordered it in anticipation of testing the Law folder with the Maxim system.
https://www.anarchyoutdoors.com/tactical-folding-bolt-carrier-extension/
This little hollow adapter costs an insane $70.
Installation:
Installation is fairly simple. Remove the rear internal bolt weight from inside the hollow bolt. Remove the current buffer/spring and insert this captured system. The buffer detent and spring can be removed as well. They aren’t needed with a captured spring sytem. This makes it very easy to maintain.
Charging the firearm & unlocking force:
9mm mechanical delay systems are biased. When pushed from the front, they resist. When pulled from behind, they don’t. With the CMMG or the MP5, the charging handle acts on the carrier which pulls the bolt’s mechanical system out of the locked state from behind, significantly reducing the force to unlock since the only resistance is from the action spring. With the Maxim, pulling the charging handle unlocks the bearings the same way as a fired cartridge, pushing it backward from the front, requiring much more force.
Charging the firearm for the first shot is noticably harder than normal. For me, it was frustruatingly difficult, and I had to brace the stock against my chest to pull the bolt out of battery. It’s nothing like a normal AR-15 or AR9. A normal AR-15 or 9mm is very easy to move the bolt with even just one pinky finger on the charging handle.
By hooking a scale on the charging handle, we can measure the amount of unlocking force needed to overcome the mechanical advantage of the rollers. This should be the same as what a fired cartridge needs to overcome to start the bolt moving. (Measurements taken with hammer cocked back already.)
- 9.5 lbs = Normal AR-15, milspec spring.
- 11.0 lbs = Gentle Recoil System, Wilson Combat flatwire spring
- 13.0 lbs = MP5 clone
- 30.0 lbs = Maxim Roller Delayed System
If someone didn’t know any better and picked up the Maxim system for the first time at a range, they would probably think the gun is jammed. This can be a problem, and I suspect it will be a deal-killer for a number of people.
The charging handle of the AR has a very small surface area for the fingers, and is not designed to give the user leverage over this much resistance. With a stock against the shoulder it’s possible for me to pop it loose when holding the left side of an extended-latch charging handle, but it’s not easy.
[See below for charging handle comparison results. TL;DR: get a Radian Raptor or Radian Raptor-LT]
Once you get over the initial “wall” of resistance, the resistance drops and it’s easier to keep the handle/bolt moving backward. It feels like it has a fairly strong spring for 9mm, more than may be necessary. [Spring measurements/specs added in update, below.]
Dot movement between shots:
This is very tough for me to judge, but some folks asked for it. More than an MP5, less than the Gentle Recoil System, a lot less than a standard buffer setup. That’s about the best I can do. I don’t shoot competition with 9mm carbine, so I don’t know what to look for.
Recoil comparison MP5 & Gentle Recoil Buffer System:
Copy and paste the above… More than an MP5, about the same as the Gentle Recoil System, a lot less than a standard buffer setup.
LAW Tactical Folder compatability (with Anarchy Outdoors adapter):
OK, but not great. The good news is that the gun did cycle just fine with all 3 uppers.
I had to unscrew the buffer tube 1 full turn or the adapter was pre-loading the Maximum RDB. Pre-loading by pushing the buffer rearwards overrides some of the mechanical delay system, so should probably be avoided.
With the Law folder, last-round bolt hold open would not work with 115gr ammo with the 5.5″ and 8.3″ upper, but did work with the 16″ upper. I tried some slightly hotter 115gr. but it still didn’t work. It may function with some 124gr. or NATO loads. The Anarchy Outdoor adapter adds 2oz to the system and that may rob enough energy from the shorter barrels that it short strokes the bolt just enough to miss the bolt catch with bulk 115gr.
Without the Law folder, LRBHO did work with all barrel lengths.
Suppressor use/port pop:
I ran out of time today, but other people have already tried it and stated that port pop and gas blowby was significantly reduced.
Range video:
For the shooting comparison at the end, watch the movement of the FRONT SIGHT.
Conclusion: Will I use it?
No. It’s much too hard to rack the bolt on a closed chamber. Can it be done? Yes, but it’s a lot more difficult than expected and will likely require changing your AR charging technique to get enough leverage. Beefy people and blue collar workers who use their hands all day will probably have fewer problems with it. Competition shooters will probably love it, but I have a feeling that, for most average people and casual shooters, it would wind up sitting in the safe, unused, after just a few range trips. [See August 2023 Update below for similar feedback from others who have now bought the system.]
I’m going to look into larger charging handes. Some folks have already stated that the Radian Raptor improves charging with the Maxim. It has thicker/longer “wings” with increased surface area. [It does! See below.] Another similar option may be the Aero Breach Large charging handle. [Purchased. Didn’t make any difference.] I was also alerted to the Springfield LevAR charging handle that uses a cam mechanism to “break free” the action, and this may be the best option. It operates from the right side, requiring you to break grip to use it, but a cam provides mechanical advantage which should make it much easier. [Works! But awkward.] These options add around another $100 on top of the cost of the Maxim unit.
The MP5 charging handle has more surface area, is in a location that allows a lot more leverage, my wife can use it easily, and the MP5 has less recoil.
The Gentle Recoil Buffer Systems are about the same as the Maxim in mitigating recoil. They’re heavier, but are very easy to manipulate and cost about the same, so I’m sticking with the GRS in my guns for now.
The Maxim RDB may be good for competition shooters with strong hands and/or with large/oversized charging handles. If you can use it, it does give very good recoil mitigation in a simple drop-in solution.
Updates:
After just 2 days I’ve already gotten responses from a few people claiming that it’s not difficult to charge and implying I must be doing something wrong or I’m a malnourished weakling. I have to assume that these folks fall into the “beefy/use their hands all day for work” category. [Sure enough, turns out one of them is a blacksmith/farrier.]
I think these folks’ experience will be the exeption, not the rule. I’m sure many people will be able to use this system, with a combination of body mass, upper body strength, and technique coming into play. I suspect that for most average people from the general population it will be frustruating, obnoxious, and they’ll get tired of it quickly.
Remember, too, I never said it was entirely impossible to use, or overcome the resistance, I said it was difficult to charge the gun and rack the bolt, and it definitely is. It’s nothing like charging a regular AR.
I ordered an Aero Breach large lever charging handle ($55 at Classic Firearms) yesterday and it was dropped off this morning:
Aaaaaand, it made no difference at all. I had a video planned, but after about 20 takes I gave up.
Suffice to say that unless you have a beefy build, work a blue-collar job using your hands all day, or have trained in a technique to overcome a charging handle with a lot of resistance, you’re proabably better off with something like the Gentle Recoil System which practically anyone can use to significantly reduce felt recoil in a 9mm AR.
UPDATE June 29:
In an unexpected affirmation that I was correct and not biased in my statements that the Gentle Recoil System has about the same recoil feel as the RDB, this was posted to ARFCOM:
I took a lower with the [Maxim] RDS system and another lower with the “Gentle Recoil” system, along with a single 16″ barreled upper, to the range this morning. I was shooting WCC 124gr +P ball ammo in both lowers.
My GR system consisted of the following parts:
JRC extended buffer tube
Kyshot RB5007 buffer
2 – Kynshot 2.5oz. spacer/weights
Wilson Combat flat-wire buffer spring
First, the RDS system worked perfectly from the first shot. As others have noted, there is considerable more force needed to charge the rifle, but I found it manageable. The lower with the GR [Gentle Recoil] system worked equally as well.
There were three shooters testing the carbine and we all came to the same conclusion. We agreed that the GR [Gentle Recoil] system was the equal or slightly better that the Maxim RDS. That conclusion was drawn using only one ammo type, so results might be different with other brands and bullet weights, pressures, etc.”
So there you go. For about the same price, and much easier to use and manipulate the charging handle, the Gentle Recoil system seems to provide similar, if not better, performance.
Charging handle force update, July 1: The pre-load significantly affects charging force.
Video explanation:
I put the Maxim system back in one of my builds and set the buffer tube so the RDB was barely touching the bolt. The charging handle took about 30-32 lbs. of force to pull back (measured with a digital scale on the handle). I had the same struggle I had before – very difficult to pull, requiring bracing the back of the tube against my body to pop the system past the “wall” of the mechanical delay.
I tightened the buffer tube 1 turn into the receiver. Now, closing the upper receiver caused the bolt to push the RDB back firmly into the tube. This pre-loaded the mechanical delay. The charging handle now took about 26-28 lbs. of pressure to pull back. This 9-10% change is enough of a difference to make a big difference. I could pull the charging handle back without bracing the rear of the tube. It’s still more than normal, but now it’s completely manageable.
This is one of the reasons why I believe some people are finding it easier to pull the charging handle back with the Maxim system in place. All my range time was done with NO pre-load resulting in maximum resistance.
Pre-loading the system will reduce the available resistance to the blowback forces from firing. Is this OK? We don’t know and Maxim neglected to provide guidance on pre-loading the RDB captured system.
I’ll take it out and do some more testing with the buffer pre-loaded when I can.
For about $56 each, I also ordered a Raptor-LT and Springfield LevAR charging handles from Dack Outdoors to test with the system and see if they make any difference. I have ordered from them Dack Outdoors before. They have very low prices but it takes about 30 days for your order to arrive at your door. Since I’m not in a rush, I don’t mind waiting.
UPDATE July 7:
Yes, I went there….
Here are the spring stats. These may not be 100% accurate, but should be relatively close:
- Length: 9.5″
- Wire diameter: 0.045″
- Active coils: 38.5
- Spring outer diameter: 0.45″
- 9 lbs. pressure at 4-1/16″ compression (bolt in battery, no pressure on buffer, assembled resting state)
- 15 lbs. pressure at 7-5/16″ (compressed to full bolt rearward)
- Spring rate (15 lbs. – 9 lbs.)/3.25″ = 1.85 lbs./in.
By these measurements, spring performance is slightly more than a Sprinco Red. This is more than 9mm blowback needs, but may be necessary for proper operation of the mechanical delay.
Weight of the roller delay mechanism and collar is about 4.3oz. The mechanical delay allows the use of much lighter masses in a blowback. With a 12oz. empty bolt, this would give about 16.3oz. total blowback reciprocating mass.
Update August 2023: More feedback is rolling in about the charging force required confirming my experience. These were posted to ARFCOM:
#1: “I guess I must be a malnourished weakling like [the author] because this is more than just a little bit of extra effort to charge the action now.”
#2 “This thing is hard to charge. With a normal mid-sized charging handle, it was tough to charge. Butt plate in center of chest and pull hard and tough on fingers. I then switched to Gibbs side cocker upper, better but still tough on fingers. You could not just reach up while butt plate in shoulder and charge the unit. It took some repositioning of the gun.”
Update August 2023 #2: I finally received and tried out the Radian Raptor LT and the Springfield LevAR charging handles (top and middle in picture).
Compared to a Vltor Mod 2 and the Aero Breach, the Radian Raptor is indeed better to use. It feels much better on the hand/fingers, making it easier to apply the force needed to pull the Maxim out of battery. The Raptor/Raptor-LT would be my choice for use with the Maxim RDB. One-handed charging is more comfortable, but it’s not “easier” force-wise.
[This is the best price I’ve found but it WILL take 30 DAYS for delivery. Be patient!
https://dackoutdoors.com/product/radian-raptor-lt-chrgng-hndl-556-blk]
The LevAR’s cam system is a very clever bit of engineering and actually reduces the charging force a lot, making it much easier to charge, but it’s awkward to use. If I were to use it with the Maxim, I would just deploy the lever arm and leave it out. If you get one, be sure to oil the heck out of it. It comes completely dry.
No, I haven’t gotten back out to the range to put more rounds through the Maxim. Hopefully soon.
Update August 2023 #3: Bought a digital force gauge with peak recording. Should be more accurate than the crane scale I was using where I had to try to catch the peak reading on camera. I used a cleaning rod to push against the bolt face down the muzzle, and paracord to pull the charging handles.
Digital force gauge readings: (for all 2 of you that care…)
MP5, standard trigger pack hammer spring:
MP5 @ bolt face:
Hammer back: ~24lbs. to push the rollers out of battery
Hammer forward: ~32lbs.
MP5 @ charging handle:
Hammer back: ~11lbs. to start the handle moving.
Hammer forward: ~20lbs. to cock the hammer.
AR9, Maxim roller delay, Aim SSTAT (Rise 140 clone) trigger:
Maxim @ bolt face:
Hammer back: ~24lbs. to push the rollers out of battery
Hammer forward: ~26lbs.
Maxim @ charging handle (Raptor-LT):
Hammer back: ~28lbs. to start the handle moving.
Hammer forward: ~31lbs.
Update Sep 2023:
Tested to see if adding round count changes pull force for initial charging.
Used a digital force meter/paracord to measure charging force (hammer cocked). My Maxim unit had about 200 rounds on it so far. No preload. 3 measurements averaged at each interval.
At start: ……. average 29 lbs.
+100 rounds: average 31 lbs.
+200 rounds: average 29 lbs.
I know it’s not a lot of rounds, but it looks to me like round count probably doesn’t have that much of an effect, if any… People saying it “gets easier” after shooting a while are probably just getting used to the charging force and the technique needed to use it.
How does it do with a suppressor?
Fired 10 subsonic rounds with 9.4oz. buffer, 10 subsonic rounds with Maxim. 5.5″ barrel, Octane suppressor.
No noticable difference in sound/smokeyness to me. Both were a bit smokey as expected. If there was a sound difference, I didn’t notice it, but I’m not a suppressor connoisseur, either.
A colleague who also performs experiments and has much more experience with delay systems and suppressors said the Maxim was noticably noisier and smokier with a suppressor. I’d tend to trust their judgement a bit more than mine in this circumstance.
Blowback9.com 