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Posted: 7/26/2016 11:49:27 AM EDT
I tried posting this in GD and got zero replies, am hoping that maybe GFD has people a little more interested in the topic.

I was wondering if anyone here has any real familiarity with the type of operating system that the AA-12 shotgun and Ultimax 100 LMG use, where the bolt carrier never fully contacts the rear of the receiver.  One of the chief selling points of both guns is the controllability in full auto due to the mild recoil (the AA-12 supposedly has a tenth of the felt recoil of a conventionally operated 12ga semiauto), but a big part of that is the inertia of the BCG moving forward to chamber a round.

 I'm curious as to how much recoil mitigation such a system might provide in a closed bolt firearm, when the only thing that would affect recoil is the spring system vs a conventional recoil spring arrangement.
Link Posted: 7/26/2016 12:06:02 PM EDT
[#1]

USAS-12 (predecessor and better designed) owner here.  It was imported as a closed bolt, magazine (10 & 20 rnd) fed, 13lb empty / 15lb 20-rnd drum loaded , ""combat"" shotgun.
Yes, the constant recoil system helps in that there isn't a hard slap against your shoulder.  But the 13-15 pounds of weight doesn't hurt either.
How much?  Well, after running through a 20 round drum, it's nothing at all to slap in another and do it again if your arms aren't tired.

Mine is semi-auto, but the 'full auto' rate of 350-rpm is easily imitated with some trigger control.

You can see the recoil spring extending from the buttstock; compressed it goes to just short of the rear of the ejection port.

Link Posted: 7/26/2016 12:13:46 PM EDT
[#2]
I have experience with my 9mm open bolt subgun (MAX-11) and my 9mm closed bolt semi-auto SBR (AR15/9mm) to draw on. Both guns works by straight blow-back, and in both guns the bolt can be adjusted to hit the back of the receiver or not, depending on bolt/buffer, spring and spacer choice.

My open bolt subgun is smoother when the bolt does NOT hit the back of the receiver, and the rate of climb when firing bursts is substantially less. This makes it easier for me to keep the bursts on a target for multiple hits, and makes the burst more controllable when "sweeping" across an array of targets. I suspect a lot of this has to do with the advanced primer ignition feature of the design - the bolt seems to move forwards and backwards in a very predictable and controllable manner. The shooter has to get the timing of recoil recovery perfect to keep the sights dropping back into the middle of the target at the moment of ignition.

My closed bolt SBR is just the opposite - although more abrupt in recoil impulse, a shortened bolt stroke makes the gun feel significantly flatter shooting, with noticeably faster sight recovery, and the speed of splits and transitions is measurably superior. I just posted a thread on this phenomenon, with data, HERE.

The results could be different with other guns, especially gas operated designs and rifle-calibers with brakes, but the above is my experience with two comparable models, and I hope this answers your question. In any case, the question is moot for civilians - no new open bolt guns for us.
Link Posted: 7/26/2016 12:44:45 PM EDT
[#3]
4thPointOfContact: I didn't realize that the USAS used the same (or at least a similar) system.  Couple of questions: How much does the spring/guide assembly weigh?  Does the spread-out push of the recoil help keep the muzzle on target vs something like a Saiga (and how much might simply be attributed to the gun's considerable weight)?  And why do you think that the USAS is better designed than its successor?

StealthyBlagga: Interesting results.  The lack of open-bolt guns is exactly why I'm asking the question - are there open-bolt-only systems that can be applied to make guns that the average American can actually buy?
Link Posted: 7/26/2016 5:38:56 PM EDT
[#4]
Discussion ForumsJump to Quoted PostQuote History
Quoted:
How much does the spring/guide assembly weigh?  
Does the spread-out push of the recoil help keep the muzzle on target vs something like a Saiga (and how much might simply be attributed to the gun's considerable weight)?  
And why do you think that the USAS is better designed than its successor?
View Quote

1)  No clue but either open or closed bolt needs a spring and guide so it would be the difference between the two and it's negligible.  I'd estimate the entire buttstock, spring, guide assembly is 2-3lbs at most.
2)  Never fired a Saiga but the USAS-12 just kinda 'floats' in midair; no real rise from recoil if you're prepared for the push.
3)  I think there are two design philosophies at work, the USAS is built like an AR (well, duh, lol) all forged aluminum whereas the AA-12 reminds me more of a STEN gun with an internal skeleton and cosmetic panels on the exterior.


For all I know, I'm just trying to justify the purchase of my most useless toy.  And I say that because of the realities of trying to schlepp around a USAS-12 or an AA-12, either one - - -

- weighs more than 12 pounds empty and closer to 15 fully loaded
- is over a foot tall with a 10-round stick magazine, slightly less with a drum
- is over 8 inches wide with a drum and 4 with a stick magazine
- drums are 8"dia. x 3" thick, sticks are 1.25x2.5x12 (basically a foot long 2x4 to give an approximation of the bulk)
- will run out of ammunition faster than just about any other main weapon in inventory.  IF the basic load is going to fit on a vest you're probably going to max out at 1 drum on the gun, 2 drums on the vest and Maybe 4 stick magazines.  100 round, max and you're basically more of a pack mule than shotgunner.
Link Posted: 7/26/2016 5:40:08 PM EDT
[#5]
Ive cut the gas in half in an ar and used a light weight bolt and it makes a hugh difference in recoil and muzzle climb.
If i cut it so much it does not even lock back on an empty mag recoil and climb is even less.
Link Posted: 7/26/2016 11:52:28 PM EDT
[#6]
When I built up my saiga 12, I tuned it so the bolt would not smash into the rear trunion for obvious reasons.

It is the softest shooting 12 gauge I have used.
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