Okay, crazy question so bear with me. Why does an AR9 have to be a blowback system?
From what I have read, the 9mm blowback has a sharp recoil (not unbearable or anything, just a heavier recoil than expected). Why not make it a gas system and it be softer recoiling?
The .223/5.56 bolt should work for the 9mm as they are the same case head size, right?

Sounds like a question for the Mad Machinist.

I do not believe the gas wave of the 9mm is conducive to direct impingement.

With a 5.56x45, the propellants are still burning and gases expanding for quite some
time after ignition, whereas the 9mm propellant burns much quicker and the gases
reach maximum expansion much sooner, and in a much smaller volume.

The gas port would need to be very close to the chamber, and the dwell time extended
to get   reliable cycling; this would cause an abrupt/violent recoil impulse, negating
the whole point.

Several people have made 9mm gas guns. The problem is that most factory 9mm ammo is incredibly dirty and much of it doesn't play well with the gas systems. The main guy who builds them and posts on here recommends hand loads with certain powders that work well and are much cleaner.

So to answer your question they don't have to be blowback but it is a lot simpler,cheaper, and very reliable with a huge range of loads so it is the easy pick for manufacturers. Sig chose a gas piston for the MPX so there are platforms out there.

The gas port is built very close but we are still talking about 9mm which is not high pressure and harsh recoiling. The owners of the gas guns say they are noticeably softer and more controllable especially in full auto.

I am by no means an expert on this but I'll take a stab at it. I'd guess that part of the issue is the bolt. Unlike a standard ar bolt, the 9mm bolt does not lock in to place. Also the 9mm is a pistol cartridge, so it's designed fro short barrels and to burn it's charge relatively fast. People have to run heavier springs and buffers just to help hold the bolt closed on the AR9s. The addition of a gas system would only make it worse. You'd have to redesign a locking style bolt to make a gas system even feasible in my opinion. I'm sure Rudy will have a much better explanation but I think that's the long and short of it.

The DI 9mm does run, after I complete a current build I will have Ron @ RMW XTREME build me a DI 9mm for my NFA small frame glock lower, should be fun. Here is a quick video of Ron's DI 9mm with the fun switch... as was stated most factory 9mm is very dirty and gums up everything fast, but if you reload as I will be for it you can use cleaner powders and eliminate most of the fouling problems.

You use a 5.56 bolt that's been reamed out slightly for 9mm when building a 9mm gas gun.  You also don't use the heavy buffers and springs that you'd use for a 9mm blowback.

Rudy,
I haven't fired mine yet, just has been an idea/question I've had for several months now.
I believe you have answered the issue in the Facebook posting this morning.
Thanks for your help and insight.

Out of curiosity and so I can get buyer's remorse since I've already sunk a couple grand into my 9mm build and it's not going anywhere, how much softer is the recoil on the MPX than a good blowback build (I'm using JP bolt and SCS)?

A bit, but it's dirtier - yep, you read that right, dirtier than blowback.  I've been a big cheerleader for a piston 9mm and grabbed a Gen1 MPX as soon as they became available.... most disappointing gun I've ever purchased.  Aside from SIG shenanigans on Gen1 vs advertised specs and then shipping incompatible Gen2, my MPX runs noticeably significantly dirtier than my simple blowback AR9 or MP5 clone.

I wanted a clean-running piston gun, thought the MPX was the holy grail....  Now I'm hoping Rudy can pull one off someday.

The MPX 9x19mm upper won't drop onto an AR lower, correct?

Correct, the MPX lower is very AR-like but not dimensionally the same.

Is anyone using a lightweight carrier in a 9mm gas gun?



 

I am.  

I have a RMW 9mm Gas upper and after building a lightweight 5.56 AR with all the light weight gas system parts (ltw carrier, adjustable gas, lightened buffer and spring), i figured i could try the same with my 9mm gas gun. It WORKS!

Since the 9mm gas block is proprietary, no change is possible there, however, Rubber City Armory offers thier lightweight carrier with the adjustable gas key. Now i could adjust my load recipe to back off on the powder, but with the adjustable key, i can keep making my current load which can also run a full weight carrier if need be. I also have a reduced power recoil spring (wolff i think) and a TACCOM delrin buffer. With these parts and the gas backed off, its hilarious how soft it is. Now i keep the gas setting high enough to keep the lock back function on the DDLES lower, but i can go even lighter if necessary. Bolt carrier only needs to recoil 1/2 the total travel to actually eject and strip fresh rounds from the short glock mag, but to lock back you have to go all the way back to pick up the bolt latch.

There is 1 weird factor with going too low, the spring tension from the magazine spring pushing on the bullet column imparts a certain amount of friction on the bottom of the bolt carrier from the pressure on it from the topmost cartridge. If the tension is high (full magazine) and the recoil force is too low from too little gas, it will short stroke and malfunction. You wont see this with testing with just a few rounds in a mag, but fill it up and suddenly its FTF city.

Your making me want to try a gas 9mm

Adams Arms makes a pistol length piston system for the AR. Would that work to operate the bolt but not foul up the action with gas?

It would be really nice if the lighter springs required for the gas operated 9mm AR allowed you to eliminate the buffer and use a short spring nested into the short bolt carrier, so you could eliminate the buffer tube and have a true pistol AR or folding stock on it. Of course you would probably be into a $1000 9mm AR.

Pistol length gas is too long for these.  The gas port is almost directly in front of the chamber

Wow! I did not realize the gas system was that short for a pistol caliber AR. Thats close.

That's why there are only "exotic" uppers which are gas.  Virtually everything you see in pistol caliber ARs is blowback.

Could you tap the gas @ the correct location, and then pipe it forward up to a short piston?

With enough money anything is possible.

The Sig has a super short piston setup

Yep, and what I do NOT know is whether the MPX is dirty because there flat wasn't any other way to make it work, or if SIG made some tradeoff and taps more/less gas than needed or whatnot.

In other words, it's *possible* that anyone else who makes a gas piston 9mm will hit the same problems SIG seems to have with respect to running dirty.  I'm not a mechanical engineer, so I won't attempt to analyze/explain - I'll just keep complaining

I'm still waiting for you to make this happen

A Browning link short recoil upper?  

Huh, color me shocked.  Where is all the crud accumulating, on the bolt carrier?  I guess I'm glad to hear holding out for an MPX would have been a mistake...  Sorry for you man...  But it's hard to understand what's going on there.

Yep, BCG and upper receiver are filthy.  All my other short-stroke piston guns look nearly clean in the BCG/upper area with fouling almost completely isolated to piston area.

Yeah the difference in my MR556 and my DI AR is night and day in terms of cleanliness.  I've got the gas turned down so low on my DI to use with low mass parts it's not bad anymore but with a standard gas block the difference was really stark.

The shocking thing is not just that it's dirtier than expected but dirtier than blowback - I can only imagine they're opening the chamber WAY too early.  I can't come up with any other explanation

Which is probably the route that would give the best results vs a DI based system. Basically put the piston gas block right in front of the barrel nut, and probably use lightweight reciprocating components.

I guess I don't get the high recoil that many seem to attest to a blowback 9mm. Maybe people are trying to run these with cheap carbine buffers instead of a full weight 9mm, or removing the tungsten weight from the bolt or something. I also shorten the bolt travel which makes the shooting feel a lot nicer. It is a different feel than a locked breech AR.