Headspace is defined as the distance measured from a closed chamber’s breech face to the chamber feature that limits the insertion depth of the cartridge. Headspace in 9mm AR’s is usually affected most by how deep the chamber is cut into the barrel. A chamber cut too short could result in a cartridge sticking too far out of the back of the chamber and an out of battery discharge (very bad!).
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. Always bring malfunctioning firearms to a qualified gunsmith for repair.
Do I need to headspace my 9mm AR?
Short answer:
No.
Consensus seems to be “if it goes bang, and the base of the fired cases don’t show signs of bulging, it’s fine”. Stop overthinking it and go have fun at the range with your 9mm AR!
Long answer:
Still “no”, with additional details…
9mm locked breech headspace
Headspace is defined as the distance measured from a closed chamber’s breech face to the chamber feature that limits the insertion depth of the cartridge.
Checking headspace can be very important for high-power bolt action rifles and semi-auto rifles where the bolt locks shut before each shot (“locked breech”). The space available for the cartridge in the locked system is critical and the cartridge case is encapusulated inside the locked chamber and breechface pocket. Too much headspace (too much room for the chambered cartridge) can result in case-head separations and hot gasses blowing into the shooter’s face. Too little, and the bolt won’t close.
The headspace measurement for 9mm is going to be the product of 2 measurements:
- The depth of the breechface cut into the bolt face
- The chamber depth
Per SAAMI, the total headspace measurement for a 9mm Luger cartridge should be from 0.754″ to 0.776″. The 9mm cartridge itself should be 0.754″, but may be as short as 0.744″. So the headspace could be up to a maximum of 0.032″ larger than the minimum cartridge length and the cartridge is still expected to fire normally (that’s about the thickness of 8 sheets of printer paper).
Based on what I can find, the typical breechface cut depth in an 9mm bolt seems to be somewhere between 0.125″ and 0.135″. Let’s use 0.135″.
Total SAAMI headspace = 0.754″ to 0.776″
…minus…
Breechface cut = 0.135″
…equals…
A chamber depth between 0.619″ and 0.641″ in a locked breech 9mm.
Now ignore everything I just told you, because a 9mm blowback AR isn’t a locked breech system, so none of that matters.
Seriously.
9mm blowback headspace
If we were working with a bolt action or locked breech pistol, all those measurements above would be critical. But we’re not. We’re working with blowback, and blowback systems don’t care about all that because blowback systems don’t have a locking breech. The system is “open ended”, allowing for some wiggle room. This is part of why blowback is a very forgiving system.
9mm “headspaces” on the case mouth inside the chamber and the bolt doesn’t lock closed. The case mouth is the brass ledge at the top of the case where the bullet is inserted.
When you close the bolt on a blowback 9mm, the bolt shoves a cartridge forward and “floats” it into the chamber. When the cartridge mouth stops on the headspace lip in the chamber, the bolt continues forward and shoves the rear of the cartridge into the breechface cut on the bolt and the extractor snaps over the rim.
This means the cartridge has to stand “proud” of the barrel face enough so that the bolt can engage the cartridge rim properly. The common recommendation is for 0.135″ of protrusion above the barrel face. This also means that the bolt face doesn’t actually come into contact with the barrel face.
Bolt stand-off from barrel face
When a cartridge is chambered in a blowback, there’s a very small space between the closed bolt and the chamber/barrel face (we’ll call this “bolt stand-off”). The cartridge is sandwiched between the breechface and the end of the chamber, but the bolt face should not be resting directly against the barrel face. It’s very close, but not touching. The cartridge is acting like a piller, holding the bolt face slightly away from the barrel face.
I found a post from a very reputable gunsmith on social media stating that they set their 9mm blowbacks so that there is a 0.010″ to 0.012″ bolt stand-off with a chambered cartridge, but as long as the bolt face doesn’t hit the barrel face during chambering, it’s fine.
So essentially, in a 9mm AR blowback, headspace on a closed bolt empty chamber is always “short” compared to the SAAMI specs.
And yes, the cartridge case can stick out of the chamber a little and it won’t blow out. 9mm is relatively low pressure compared to centerfire rifle cartridges. The cartridge base is thick and the walls gradually taper internally to the case mouth. The thick base can contain the pressure of firing provided there isn’t too much of it exposed outside the chamber.
Problems
If headspace is too short (short chamber) in a blowback, there may be an excessive amount of unsupported cartridge sticking out the back of the chamber. This can result in the cartridge base bulging or rupturing. If it ruptures, the cartridge base bursts, releasing high pressure hot gasses and brass fragments. This usually damages the magazine and may damage the firearm and the shooter. By the way, the vast majority of burst cases in blowback 9mm AR are caused by bolt bounce or overpressure issues, NOT headspace (chamber depth) problems.
If the chamber is cut too deep in the barrel (headspace too long) the extractor may not have room to snap over the rim before the bolt face contacts the barrel face. Because the case head is now too far away from the breechface, the firing pin may not be able to reliably strike the primer. This can result in light primer strikes and failure-to-fire issues. There are documented cases of this occuring when a barrel maker cuts the 9mm chamber too deep.
Before anyone mentions it, 9mm cartridges don’t headspace on the extractor. This is another one of those myths that just won’t die.
Headspace gauges
As you’ve probably been able to guess by now, headspace gauges don’t really work to test a blowback. Don’t waste your money on them. Headspace gauges check for excessive chamber length. For headspace gauges to work, you need to be able to close and lock the bolt. Blowback bolts don’t lock closed, so they’re essentially useless in a blowback system.
How long should the chamber be?
Since the blowback 9mm AR headspaces on the case mouth at the end of the chamber, let’s use the shortest acceptable SAAMI case length and do the math.
Example:
Breechface cut = 0.125″ (this is the typical normal cut depth)
Minimum cartridge length = 0.744″
Bolt stand-off = 0.010″
This should put our chamber at 0.609″
So what does this all mean?
I just measured one of my 9mm chambers and it is roughly 0.605″. Both my 9mm bolts have a breechface depth of roughly 0.135″, so they’re a little deep. This means with the longest 9mm cartridge SAAMI length of 0.754″, this gun would have a bolt stand-off of 0.014″ (equal to about 3-4 sheets of printer paper). The shortest permissable cartridge length of 0.744″ would have a bolt stand-off of 0.004″ (one sheet of printer paper). Reports are that most actual 9mm cartridges in use today tend to be closer to 0.745″ on average.
So by all accounts, I have the correct “headspace” in this blowback 9mm even though it is well below the SAAMI official “headspace” standard for 9mm.
I have never had any problems with bulged cartridges or OOB discharges with this gun, including with full power NATO ammo. It performs exactly as expected.
Now stop overthinking it, stop trying to measure your headspace in a 9mm blowback, and go have fun on the range with your 9mm AR! As long as it goes “bang” and the fired cases aren’t bulged, you’re good!
Blowback9.com 