What is the simplest spring system that could produce recoil absorption comparable to that exhibited by the AA-12 assault shotgun?

In this video, the video emphasises the very low recoil level of the AA-12 fully automatic shotgun. In particular, at this point in the video the presenter uses one hand to relatively lightly grip the gun while firing 300 rounds per minute. Note that the presenter does not brace the shotgun against his shoulder.

The image below is a frame from the video at 5m:03 - it shows the gun in"mid burst" - two cartridge cases can be seen in the air, one having just been ejected and the other is closer to the camera. The users 'stance' appears to be intended to show the effortlessness of holding the gun while firing. If he is in fact using typical 12 gauge shells (which he implies that he is), holding a "normal" shotgun like this when firing would result in it being torn from his hands.

In the video the presenter claims that the shotgun exhibits, for all intents and purposes, zero recoil. He remarks that there is a spring system in the buttstock that counters the momentum of the recoil.

I can think of two ways that the spring system could work:

1. The first recoil (or something else) compresses the spring, and then the spring extends upon the generation of the second recoil.
2. Each recoil compresses a stiff spring that extends more slowly than the recoils compress it. Assuming a sufficiently quick firing rate over a sufficiently protracted period, complete compression of the springs would obtain; consequently, the user would need to abstain from firing the shotgun until the springs extended, lest s/he absorb the recoil herself.

However, it seems to me that those systems would work only if the user braced the shotgun against something (i.e. his/her shoulder).

What is the simplest spring system that could produce recoil absorption comparable to that exhibited in the video?

• The video does not support your claim of "limply grip", nor "zero recoil". So there's not really a question here, once those are removed, is there? Jun 22 '15 at 20:46
• A cursory review of the gun's Wikipedia entry would have done a great deal to correct the faulty premise of this question.
– Air
Jun 22 '15 at 21:29
• @Air He could have done better (eg read the Wikipedia article :-) ) BUT the question is still reasonably valid if you substitute a number of web descriptions for his one. One site simply says it uses a much longer spring travel than most - which would certainly help, but . the energy still needs to be dealt with. I was interested in how much energy is involved - and the 150 Watts continuous while burst firing (or more like 500 Watts with magnum loads) is more than would be tolerable without energy dissipation. So ... Jun 23 '15 at 0:23
• IMO that is far from a "limpy grip" you can see the tendons straining to hold it and after the magazine is empty you can see him pushing the gun forwards until he catches himself. Jun 23 '15 at 8:16
• @Hal I've edited your post to (hopefully) better reflect what is seen while still highlighting the very significant reduction in recoil that they AA-12 exhibits. I've also added a frame from the video. Feel free to revert the post or edit the edit as desired. I strongly suspect that he is cheating (eg the 'holes in the door' demo suggest very low energy shells) and his accounts of the recoil do not match what you see and hear elsewhere. BUT it is still a good demonstration of the gun's reduced recoil and the question is still well worth asking. Jun 23 '15 at 15:48

The simplest, and possibly only, spring only system that would work like that would be made of nonexisteum as the energy would still have to be dealt with, and the power level exceeds what is seen in the video.

This video TFB provides another firing demo which gives a more realistic impression of the recoil level experienced in practice. The shooters take care to place the butt to their shoulders in a conventional manner, even after having fired a number of shots, and the recoil reaction on this bodies can clearly be seen. While the recoil may be substantially lower, it is nowhere nearly as low as "zero recoil" - and also is much lower than you'd expect from the initially cited video, suggesting that he used very low energy loads.

However, a spring plus 'other things' to spread the energy pulses and absorb as much energy as possible would work - and demonstrably does, as the (US, not Russian) AA12 is an entirely real gun.The original AA-12 was a 1972 US design by Maxwell Atkinson (images). It has since been developed by 'Military Police Systems' into the current version.

When the bolt flies back after firing to cycle another round, around 80% of what would normally be felt as recoil is absorbed by a proprietary gas system. A recoil spring grabs another 10%, leaving the final recoil a remarkable 10% of the normal recoil for a 12-gauge round—so you can point the AA-12 at a target and unload the full magazine without significant loss of accuracy.

Based on the notes below it seems likely that the "proprietary gas system" not only absorbs and stores the energy initially, but must also dissipate energy - presumably by heat transfer via compression. Power level to be dealt with during during firing at full rate of 300 shells/minute is about 150 Watts, or about 600 Joule for a maximum of under 4 seconds for an 18 round drum magazine. 600 Watt.seconds (600 J) is enough to make the gun 'somewhat warm' but low enough to be dealt with realistically.

If the level to be dealt with by the user is 10% of total, as claimed that's a continuous rate of about 15 Watts. That's a level of power input that people would exert for short periods on many hand operated units that take noticeable user power (many not so well known today) - sirens, power generators, window winders ... . Well below the level needed to crank a car by hand (remember those ? :-) ). 12 gauge load energy can vary significantly. Looking at the effort taken in the video I'd guess his loads were not up to maximum power level for 12 gauge.

Notes:

Overall the user + gun must absorb energy from the explosive charge as they apply an equal and opposite reactive force to the accelerating projectile. The velocity of the overall mass (user + gun) will be low (fortunately) compared to that of the projectile(s). The timing and shape of the reaction can be altered by springs (gas or mechanical) and moving mass but the "reactive energy" must be absorbed either as heat + user absorbtion.

Watching the video portion in question it appears that the gun barrel dips during the period during and after firing rather than rising as I would have expected. This could be achieved by accelerating an internal mass using pre-stored energy (gas, springs, ...) to counterbalance the impulse peak. The energy still needs to be absorbed but with suitable design could be spread across the available time period linearly or in a manner that best suits the users muscle response.

Energy involved varies with load used and can differ by more than 5:1 - so what you see in the video and what you experience with a maximum load can vary considerably. This reference The popular 12 gauge indicates that the upper limit that most recreational shooters can tolerate from a 12 gauge shotgun is 21 foot-pounds of recoil energy*. That translates to about 3 kg.m or slightly under 30 Watt seconds.
That's not as arcane a unit as it sounds because - the 300 rounds/minute firing rate = 5 rounds/second or a continuous power level at 30 W.s per shell of 30 x 5 = 150 Watts. Providing a sustained 150 Watts is a substantial workload. Most users on an exercise bike would be unhappy to maintain that level for more than a few minutes.

*I took the recoil energy, projectile mass and muzzle velocity figures they quoted on that page and compared calculated kinetic energy of the projectile with recoil energy. I had expected them to correlate approximately linearly but the claimed recoil energy increases much more rapidly than I'd expect from the calculated projectile energy. I tried various values of x in 0.5 x m x V^x without getting a reasonable fit. Using impulse (delta mV) is worse. Interesting. Maybe a better understanding of acceleration profile and burn dynamics is needed. Or, no doubt easier, just measure it.

Related:

Gizmag

World guns.ru More history and tehnical details.

• Can you please provide a reference for the nonexisteum spring material that you cited? I checked all the usual search engines, but I wasn't able to find a reference to that material. I would like to find out more as it sounds pretty incredible if it can replace a proprietary, pneumatic system. :-)
– user16
Jun 22 '15 at 21:58
• @GlenH7 I found a number of web references to nonexisteum (or non-existeum) but, rather to my surprise, I found the majority of them were in material originated by me :-). This was in fact a surprise as material with such properties has been discussed in various forums and papers over decades. Maybe a slightly different name is used? However, this writer talks about nonexisteum and obviously has a very clear understanding of its key properties. Jun 23 '15 at 0:35
• Thanks for the reference! The closest I had been able to find so far had been this.
– user16
Jun 23 '15 at 0:44
• @GlenH7 Yes. As in :-). That's what I meant by "maybe a somewhat different name was used" BUT I had not realised that I was the major internet source of my version of the name :-). I've used it for years so there will be a few more refs somewhere. As the > 50% source of references, does that make me 'an authority' (or an idi....)? Hmm Maybe don't answer that. .|| You need unobtainium for the limply gripped one handed AA-12. Jun 23 '15 at 3:32
• perhaps good to know is that a Watt.second is exactly a one Joule (by definition) Jun 23 '15 at 8:13

There is a pervasive misconception that recoil can be "absorbed" by such things as springs and proprietary gas systems. This is not true; eventually all the recoil produced by launching the bullet must be absorbed by the shooter. The only thing that a spring/gas system can do is spread the recoil over a longer period of time, so that the force acting on the shooter's body is reduced.

In physics terminology, recoil is analogous to momentum, not energy. Momentum is the integral of force with respect to time, $P=\int{F \cdot dt}$, and momentum is a conserved quantity; i.e. it can't be destroyed. The bullet and escaping propellant gas gain momentum during the launch because they're subjected to a huge force for a very short period of time. As a consequence of Newton's third law of motion, the gun and shooter gain the same amount of momentum as the bullet, but in the opposite direction. To dissipate this momentum, you can either absorb a lot of force in a short period of time, or absorb a lower amount of force over a longer period of time (preferable). For automatic weapons, an ideal recoil system would dissipate the momentum to the shooter in such a way that they would just feel a constant force pushing on them; i.e. no impulses from the shots.

To answer the question, the spring system to do this would be a progressive-rate spring combined with a reactive damper. This type of setup is used on some high-end car suspensions, but I imagine it could be adapted for firearms as well. The trick would be to adjust the reaction of the spring and damper so that they reduce the force exerted on the shooter to a minimum.

• I just refound this Q&A while web searching for non-existeum - which term I seem to be a major inter-web source for :-) |. A comment 4.5 years on: I agree with your assertion re pervasive misconceptions, but suggest that, as per my in answer comments " ... However, a spring plus 'other things' to spread the energy pulses and absorb as much energy as possible would work - ..." is correct. ie the spring system is used to change the characteristics of the energy transfer and then "other things" can help absorb SOME of the energy. ... Jan 10 '20 at 4:05
• ... This could be via resistive dissipation or air expulsion via suitably sized orifices either to atmosphere directly or into a vented chamber which reduces velocity peaks (somewhat). It would be interesting to calculate the difference between the (claimed) upper tolerable recoil absorption level of 30 Ws and the energy delivered to recoil by the system. Jan 10 '20 at 4:06
• You're correct that springs, gas systems, etc. can be used to spread out the recoil force over a longer period of time, however there's no way to absorb or dissipate any of the recoil momentum from the bullet such that it just goes away for good; eventually you have to transfer exactly 100% of the recoil momentum to the shooter. The only exception is so-called recoilless rifles like the Carl Gustav or Davis guns like the AT-4CS which bypass the shooter and dump most of the momentum into a countermass. (I think this is what you meant by "air expulsion") Jan 21 '20 at 17:02
• I have not looked at the "time constants" involved, which are crucial. Energy can be "dissipated" by converting it to heat (and here a trivial amount to sound and light). Here that could be by friction, by "stirring" a fluid and/or forcing it through an orifice, or by compressing air and forcing it through an orifice. You end up with hot "stuff" which either immediately 'leaves the building' in the case of gas compressed and released to atmosphere, or as rising temperature to be transferred out in a more leisurely fashion. If using water as a heat-sink you can store just over ... Jan 21 '20 at 20:58
• ... 4 Joule per cc per degree c rise, so a say 70 c rise (30c to just below boiling) gives 280 J/cc - So say 100cc of water can absorb around 28000 J without boiling! Doing it in a short enough period to be useful would be 'the trick'. Air has about 1000 times less energy capacity than water per mass. So heating air by forcing it through an orifice easily gets you large temperature rise, which assists in achieving energy transfer into say steel or other metal. The time constant of heating is minimal but that of transfer to eg metal isn't. Energy can also be stored in a compressed then latched Jan 21 '20 at 21:04