9mm AR-15 question

[Madcap_Magician]

I have always been curious- what, if any, is the mechanical reason that 9mm AR-15s are all blowback operated instead of maintaining the gas system?

The M16A4 was the first centerfire rifle I ever shot. Last summer for Law Enforcement Skills, I did AR-15 training with Colt 9mm carbines. I noticed that the 9mm carbine actually had noticeable recoil. Not much, but more than a 5.56mm carbine. I attributed this to the blowback operation versus the direct gas impingement system given my experience with blowback-operated handguns.

Nobody has ever been able to tell me why it's not practical and/or desirable to do a gas-operated pistol-caliber AR.

Anyone know?

 

[WestKentucky]

Powder is very fast in pistol cartridges so you have your main propulsion in the first few inches of barrel. By the time it gets to the gas port you probably wouldn't have the gas volume needed to operate the dgi system. Rifle powders burn slower so they are still burning and expanding when at the gas port.

 

[dprice3844444]

compare the weight of the bolt carriers

 

[Panzerschwein]

I'd imagine it would be tougher to make the gas system balanced properly. The 9mm is a lower pressure round with much less powder than 5.56, I think it is just easier (much easier) to make these guns in blowback. And yes, various 9mm blowback weapons have a big heavy bolt and when this flies backwards that is the recoil you're feeling.

 

[dprice3844444]

http://www.amazon.com/s?ie=UTF8&page=1&rh=i:aps,k:ar15 recoil pad

ar-15 recoil pads

 

[Jim Watson]

At one time Olympic offered a gas operated 10mm, but I never heard of a 9mm other than blowback.

 

[Sunray]

"...9mm carbine actually had noticeable recoil..." Something horribly wrong with that. Shot an M-16K years ago at Second Chance, there was little to no felt recoil on FA.

 

[maxxhavoc]

As has been said, 9mm pressure is nowhere near 223 pressures at the gas port.

Carbine gas port is near 6" of barrel, Middy at 8", Rifle near 12"

Here is my 223 load (barrel inches are the bottom legend, pressure is the red line:


9mm, same chart:


So with even a carbine length, .223 pressure is near 27,000 PSI at the gas port, 9mm is at 2,500psi or so. There is just a lot more pressure to work with on a .223.

 

[Madcap_Magician]

I knew the pressure differences were huge, but it seems like you could still lighten the buffer weight, buffer spring weight, and adjust the gas tube position to make one work in a pistol caliber.

 

[barnbwt]

There's another reason, too. People have done DI 9mm builds (you do need the gas block like 2" in front of the barrel nut ) but found the 9mm cartridge is very unsuited for the AR bolt head due to its very short length. When cycled, the push-button ejector will cause the round to tilt before it clears the locking lugs, and it will drag/bind/hang on them to the point of tying up the gun. And if the case does make it past that first obstacle (when fired, it moves to fast to get hung up) it tends to exit the bolt face early for the same reason. It will tilt so far it either falls off and jams, or stays hooked to the extractor but has no more ejector button travel left to throw it clear when it comes to the ejection port.

People found a way to keep the round from hanging on the lugs, by putting a spacer in where the extractor 'lug' would normally go (and grinding off the extractor 'lug') but the other issue remains, and there's not a ton you can really do about it besides cutting the ejection port farther forward (so the round's early departure from the bolt face will allow it to leave the gun)

The SIG MPX is a gas-operated 9mm platform, but short stroke piston operated (the main benefit of which is less overall gas volume is required to get force imparted to the bolt carrier than DI, and it is somewhat self-regulated by the piston's limited travel). It does, however, use the same barrel-extension locking concept and bolt design, so I assume they have found a way past these same issues that doesn't involve different lockup or a fixed blade ejector.

TCB

 

[HOOfan_1]

I knew the pressure differences were huge, but it seems like you could still lighten the buffer weight, buffer spring weight, and adjust the gas tube position to make one work in a pistol caliber.

There is a 9mm buffer...maybe the gun youbshot didn't have one...but the buffer is heavier due to the blowback action

http://www.brownells.com/rifle-part...s/car-15-m4-9mm-carbine-buffer-prod44738.aspx

 

[Arizona_Mike]

There is a 9mm buffer...maybe the gun youbshot didn't have one...but the buffer is heavier due to the blowback action

http://www.brownells.com/rifle-part...s/car-15-m4-9mm-carbine-buffer-prod44738.aspx

I like a longer buffer for 9mm too so the bolt can't get a running stop at the LRHO parts.

Mike

 

[dprice3844444]

there are several setups now for 9mm guns.brownells sells adapters for ar-15 lowers mech-tech systems uses glock pistol lowers,and there is several that make ar-15 lower setups using glock mags

 

[Madcap_Magician]

There's another reason, too. People have done DI 9mm builds (you do need the gas block like 2" in front of the barrel nut ) but found the 9mm cartridge is very unsuited for the AR bolt head due to its very short length. When cycled, the push-button ejector will cause the round to tilt before it clears the locking lugs, and it will drag/bind/hang on them to the point of tying up the gun. And if the case does make it past that first obstacle (when fired, it moves to fast to get hung up) it tends to exit the bolt face early for the same reason. It will tilt so far it either falls off and jams, or stays hooked to the extractor but has no more ejector button travel left to throw it clear when it comes to the ejection port.

People found a way to keep the round from hanging on the lugs, by putting a spacer in where the extractor 'lug' would normally go (and grinding off the extractor 'lug') but the other issue remains, and there's not a ton you can really do about it besides cutting the ejection port farther forward (so the round's early departure from the bolt face will allow it to leave the gun)

The SIG MPX is a gas-operated 9mm platform, but short stroke piston operated (the main benefit of which is less overall gas volume is required to get force imparted to the bolt carrier than DI, and it is somewhat self-regulated by the piston's limited travel). It does, however, use the same barrel-extension locking concept and bolt design, so I assume they have found a way past these same issues that doesn't involve different lockup or a fixed blade ejector.

TCB

Ahhh that makes a lot more sense. So basically there's no reason the system wouldn't work, but since the AR-15 design is built around 5.56/.223, trying to force the much shorter COL and lower pressure of the 9mm into the same scale rifle creates more malfunctions.

... so basically in order to optimize the AR-type rifle for pistol calibers, you would need to shrink the entire gun, not just swap parts.

Also, I am in desperate need of an SBR'd Sig MPX.