Overkill.
I want to preface this by stating that anyone can use these ideas freely, but I am in no way responsible if you break any laws or get arrested, hurt yourself, others, whatever.

Also by reading this article you hereby consent that if you build one according to the designs posted here you agree to let me shoot it.

It started a couple years ago as a “what if” idea. I wanted to do a saiga-12 build, and I wanted a semi-auto rifle that had knockdown somewhere close to a 50BMG yet could be fired from the shoulder. AND THEN I decided that those two ideas were not separated far apart.

This became the Mjollnir-12

The concept was a semi-auto magazine fed 12 gauge, that fired full sized projectiles down a heavy rifled barrel. Effectively a .729 caliber rifle (we’ll just call it .73cal) An obvious problem is that this places the gun in the DD category, even though it has less “destructive potential” than a 50BMG. All the rounds would be handloaded into full brass casings that had been seated with 50cal primers. I even tracked a source that could pour copper bullets around my .10in steel core rod, and I planned on turning them in a lathe to deburr and remove the excess core.

First we were going to take a Saiga-12 and try to replace the barrel, possibly adjust the recoil spring as needed for higher case pressures. A Fine tune-able gas valve was added, and I wanted to make a Barrett style comp (like you can find on the silly mossbergs) only have it in a “V” shape for both backward and upward direction of gasses.
I eventually scrapped the idea of a custom muzzle brake as mostly showing off and settled on something more “conventional” from Tromix.

Now here is where it gets different. I designed an entirely original butt-stock mounting platform that bolted to the receiver. It allowed for a standard M-4 buffer tube for the mounting of any aftermarket stock, but also raised the line of the stock above the barrel. In addition, it allowed for approx. 1in of travel between the stock and the receiver allowing for recoil compensation. Mounting the stock line above the barrel eliminates muzzle rise on recoil and when coupled with a recoil absorbing spring in the stock adapter and good gas compensation on the barrel it should reduce nearly all of the heavy recoil making it easily shoulder-fired.
Mounted forward of the shock absorbing spring is the pistol grip assembly. the pistol grip and trigger guard were to be mounted in an assembly that would lie flush along the bottom of the receiver with the trigger extending into it. Bolts inserted into slots on the grip assembly would hold the entire stock section to the receiver. The slots would allow for only .25in travel forward towards the barrel, and held in place by a compression spring inserted into the rear of the receiver and extending out approx .25in. When fired the spring would be compressed and receiver and trigger would travel into the pistol grip assembly, thereby removing finger pressure on the trigger and allowing the trigger to reset. The trigger assembly spring would be lighter(softer) than the recoil absorbing spring assuring its compression first.

Thats before I read the ATF ruling for the AR bump fire stock, I thought they had a spring and it turns out, you have to use your hand to push the rifle forward for it to bump.http://www.slidefiresolutions.com/do…val-Letter.pdf

Most of that is entirely beside the point.

To make it work with a .73cal round with an overall length not greater than 3 inches would require a custom barrel and would not fit the Saiga trunion. So I “re-tooled” the design to have a machined aluminum receiver, a hinged rear cover(similar to Texas Weapon Systems rail) also giving it a “flattop” and integrate the recoil absorbing springs into a sealed area. The result would be essentially a huge SCAR, with the infallible Kalashnikov internal operation, firing 12-gauge rifle rounds. Estimates put this model attaining a ballpark of 300m of usable trajectory, with 540gr (lead)projectile. The copper bullets would offer a considerable range and performance advantage as well.

All that, firing from the shoulder, while walking or running.

I drew up designs for double stack 20 and 24rd magazines, was working on a triple stack, and trying to figure out a reusable belt-fed configuration.
I figure the best way to carry ammunition for these would to have a belt in a reusable polymer container, and a feed mechanism that allows the belt to feed back into the container, then the container would attach similarly to a magazine, a round would be chambered and it would essentially create a giant mag. 12ga Drums are crazy expensive, and very low capacity for the weight and size, I do not hold them in high regard.
Another huge drawback is the maximum ammunition a soldier can carry. You’d run out of space fast, and the ammo is HEAVY. This gun, like the AA-12, would be the first to run out of ammo, every time.

Very early Saiga based mock up image.
The Mjollnir-12 would have been the biggest, and one of the meanest guns, suitable for anti-vehicle and SDM work, but not really outperforming the 7.62x51NATO round. In big game takedown, its better, with less range (and less meat afterward)
With specialty ammunition it could possibly beat the 7.62 nato in some areas, but never really outclassing anything that’s already available.

I was going to do a few of the stage one Saiga mods, just as a proof of concept for myself, but then this year prices started skyrocketing and it became less cost-effective.

Anyway, I can take most of these ideas and apply them to a FAL for about the same (higher) price with far more effectiveness.