Go ahead and call bullshit. Better, ask him how much he's holding over first. It's going to be on the order of 400" or 33'. If the rifle is scoped, it's going to be 40 MOA. If he says five feet, you know he's full of crap.
8" is .8 MOA. I know nothing about Mosin's, but even if they are sub-MOA rifles at 100, it takes an extremely skilled shooter to duplicate that at 1,000.
I have $1,000 says he can't hit once in ten tries.