First things first...what exactly are your intentions?
Why do you want a "50mm scope"?
Why do you want a 7.62x54R?
You are clearly just getting into this. That's obvious by thinking you can plop one cartridge in a gun designed for another. Do you think you're going to put some humongous scope on a cheap rifle and shoot 30 year old machine gun ammo through it and have a "benchrest rifle"? Or worse yet, a "sniper rifle"?
Doesn't work that that my friend. If you want to play the game of making little cloverleafs downrange, the first thing you need to invest in is some literature...like a good loading manual, because "match" ammo is really expensive
. A number of manufacturers make nice, accurate bolt guns and a couple hundred bucks can buy you a decent enough scope, and it's not going to be a "50mm". Accuracy is most about the shooter, of course, but beyond that, the ammo plays a bigger role than the rifle. You can load ammo that will make that $79 Mosin (with a decent barrel and bedding) shoot great. You can shoot Wolf through whatever you want and it's not going to give you better than a few inches.