I can confirm that this is indeed a problem in the DayZ:SA. I have myself died because of this bug a couple of times.

It doesn't say anything ingame that this gun shoots 7.62x54mmR. There is no problem to rechamber a mosin to shoot 7.62x51mm, which is a more common round to shoot. So maybe that is the case, the gun is rechambered to fit the standard NATO round. But I still agree with you if this is a fail by the developers, then they should correct it. But maybe they want to add snipers like the M24 or the M40 into the game, and these might also use the 7.62x51mm round, maybe this round is intended as a round for sniper rifles, so to keep it simple they only have one round for all of them.