OK, it’s a grainy shot, and it’s a rare gun; but there is absolutely, positively enough here to ID it.
Answer in the comments. The first correct answer gets… bragging rights. If we aren’t getting answers we’ll extend some hints, from the arcane on. We’ll ID the weapon by day’s end (if not before) and have a few more comments about it, including our assessment of why this weapon was important, and provide a link to some excellent photos. That stuff will be all below the fold (for the edification of the long tail of future guessers).
OK, we didn’t get the ID in by day’s end, but the weapon is, as Neutrino Cannon and several others guessed, a Soviet PTRS-41 anti-tank rifle, firing the 14.5 x 115mm round. MAS-38 was an inspired guess, and the trigger area in particular of this old Ivan cannon resembles the MAS, but the French SMG had less of a discontinuity between the receiver and the stock, an arrangement contanining a buffer in a tube that extends from the receiver centerline, much like the AR-10 and its many spawn. (The kink of the MAS was that the receiver centerline, unlike its blowback SMG contemporaries, was not the same as the boreline but was set at an angle).
The PTRS resembles nothing more than a massively scaled-up SKS-45, which tells us that the later SKS is really a midget PTRS. Major design concepts and even design details like the magazine catch and follower are similar, and if you look at the picture, you can see how the receiver cover resembles the one on the common postwar semi-auto carbine. Here’s a much better photo of the PTRS, where you can see clearly the SKS relationship:
The bolt on this rifle is locked back, just as it does on the SKS. The PTRS trigger is unique, very long in travel, and spongy. You can see that the PTRS lacks the stripper clip guides on the bolt carrier that are what helps one load an SKS’s semi-fixed magazine. In fact, the very similar semi-fixed mag of the anti-tank rifle loads completely differently! An en-bloc clip similar to that of the M1 rifle is used, but unlike the top-loading M1, the PTRS loads from the bottom.
The gunner releases the mag release (forward and below the trigger) by pulling it to the rear. The sheet-steel magazine housing is hinged at the front, and it and the follower (also hinged forward) pivot down. Ivan shoves in the en-bloc clip, and closes the magazine.
There is no external bolt catch on either of the Simonov weapons. The bolt-hold-open is pushed up by the follower once the magazine is empty, and then is wedged in place by the bolt carrier. To release the bolt, the operator tugs the bolt handle lightly, then releases it, and it slides home, stripping a round from the magazine into the chamber. When there are no more rounds, the bolt will lock back. To close the bolt on an empty magazine, the follower must be depressed by hand, or the magazine catch released to allow the follower to fall out of engagement with the bolt hold-open. Then the bolt is tugged back and released to release the hold-open and close the bolt.
Describing it in words like that is kind of awkward and involved, but the actual crew drill is easy and simple. The PTRS was reliable, but was soon obsolete as tank armor got thicker. Its cartridge found a new purpose in a series of anti-aircraft guns that are manufactured to this day, but that’s another story. The SKS was the final evolution of the Simonov system and it and the PTRS were vastly improved in ergonomics, robustness, and reliability over the ill-fated AVS-36.
The excellent close-up of the PTRS is from the fantastic photo essay / walk-around at gunpics.net. Highly recommended, intimate look at this rare blaster. The original, grainy “guess me” photo is a screenshot from a “WWII in Color” video.