Since I bought my Mosin-Nagant, before Obama banned their importation, I've done one heckuva lot of modifications to it ...
I started with an Archangel polymer stock, and added a Timney trigger (I asked the gunsmith at the factory to keep the trigger pull at no more than 1 pound of pull ... heck, you breathe on the thing and it triggers). One reason to buy the Archangel stock is that it is designed to accept the Timney trigger housing, which comes with a modern safety. Those who know this rifle realize that to put the original on safety, you have to pull a knob at the end of the receiver all the way back, then twist it to the left. I'm getting a little old for the heavy spring. I like just using one finger to pull the trigger's safety back. The stock has three mountings for the AR-15 mountings, and I use two of them, and also have an AR sling.
I've added a forend Picatinney rail, to which I'm mounted a good, solid bipod. You have to manually move the legs on the bipod. In other words, it's quiet, and above all, a sniper wants to be quiet.
Next week, or thereabouts, I'm adding a decent mount (with rail) and a fairly good scope (I don't have $4000-5000 for a scope!). I like the idea of having anywhere from 5-8 inches of clearance between my eye and the rear of the scope. I can shoot with both eyes open, and this adds situational awareness. The Russian ammo had a bullet weight of about 185gr. However, I'm going to use a newly-manufactured 165gr round round. The bullet will be fast, and I should get great shots at from 800-1000 yards.
I'm going to have to find out whether I can legally cut about 4.5" from muzzle, re-crown it, and add a recoil suppressor. If I can't do that, heck, I'll still be able to mount my bayonet! At heart, I really like sticking things with a sharp blade. In the field, I carry about 4-5 blades, besides a pistol. My favorite, of course, is a KaBar!
Nothing like a good old-fashioned bolt-action. Oh, did I mention that the stock mounts 10-round removable magazines? My combat load is 110-rounds in 11 magazines.