On a 91/30, the rear sight assembly is mounted to the barrel on a dovetail. If you drive out the two pins that attach the rear sight, heat the sight base gently with a small torch(the base is soldered on with a tiny dab of solder), then tap the base forward towards the muzzle, it will come right off. You'll have to remove the top handguard to do this.
Under that sight base is an approx. 3/8" dovetail milled right on top of the barrel. Clean that up with a file, order a Leapers offset air rifle/.22 scope mount
http://www.opticsplanet.net/leapers-accushot-30mm-bi-direction-integral-offset-mount.html , and an NcStar high-eye-relief scout scope from eBay
http://www.ncstar-sales.com/proddetail.asp?prod=NC_SPB432B , and you're good to go. This is the Leaper's mount, it's under $15.....
Typical NcStar pistol scope, 4x32. Many to choose from. This one's about $37.....
With a high-eye relief pistol scope, you can see the full field of view from up to a foot away, so you don't need a special bolt with a bent handle, and you can still load the rifle with charger clips.
Lots more info here on this forum.....
http://www.gunandgame.com/forums/mosin-nagant/ , just search for "scout scoping the mosin nagant 91/30", there's several threads. Lots of photos too.
Photo of the dovetail (these type mounts don't work too well, the Leapers gets a better bite)....
I've done all this to my 1938 Tula 91/30, but haven't gotten it to the range yet. It's easy to do and not very expensive.
Again, the single clamps don't grip as well....
And the mounts that just replace the rear sight leaf will work, but poorly.....
I "think" this is a Daryl's mount, supposed to be better than the eBay mounts, but they're not cheap....