Buy an M91/30 Sniper model from AIMSurplus. Comes with a scope and mount,
Failing that you can use any scope that you can find a way to mount. Affixing a scope mount to an unmodified 91/30 involves drilling and tapping. You will probably need a gunsmith.
This mount is a solid design that doesn't move even with the kick of a 91/30. The next issue is the scope that where the quality of scope and the amount of money spend come to play. You will need along eye relief scope as it sits very far forward on the rifle. Do not buy a scope designed for a 22 rifle it will not stand up to recoil of a 91/30.
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You will have some work to do. I guess it will be a project gun.
I have a PU sniper and its 1MOA accurate but after I did some work on the inside of the stock (re : Smith-sights.com article).
I did the same for my 91/30.
I got a no drill mount and tried it with a red dot and it was an improvement over fixed sights. I did not have a long eye relief scope (required for this kind of mount). The Red Dot was interesting and more accurate but I pulleed it off after 2 range trips. Removing the rear sight was a pain even to punch it. and getting it back on was even more painful on mine. Look around for vids or other articles, but if you are the kind that wants to do this to a milsurp, removing the entire rear sight exposes a dovetail (3/8" I think) where a scope rings can mount to. I"ve seen 2 people at the range who did this.
The Mosin barrel is sloppy and people who want the most accuracy out of it reload (which I did). IF you are shooting milsurp ammo and you wan to put a scope on it to make it more accurate without changing the stock, I'd guess your gonna get about 2MOA out of it. Maybe better, maybe worse. But I'm laying odds that this is going to take time and range trips to tweek it right. I just never head or talk to anyone who pulls one out of a crate, throws a scope on it and got it shooting 1MOA without spending time on it (I"ve only been talking to people since Jan on this though).
As you might be doing already, research all that your going to spend before doing the work. See if you can find a vid or page on the mount you want to buy. Ask people what kind of accuracy they are getting out of theirs and what ammo they are using.
In order of best to worse accuracy, this is what I found:
(Custom Handloads, PPU, Russian White Box(wolf), Russian/Bulgarian surplus(corrosive))
There was a guy a while back that went all out and reworked the bolt, trigger, after market stock, supressor, scope. It was no more accurate than my PU sniper was and seemed to string with the heated barrel a bit (expected).