i have found this in my search for a good mount.

its the base of a PE sniper scope mount.
i hear you can not mount standard weaver or dovetail rings on it.

i havent bought it yet. can some1 plz tell me why the standard rings people use dont fit?
is it because its between 11mm and 20mm?
is it because there is no sideways notch for the set screw?(easy fix)
can someone that has one measure the width of the rail on the top?

in the end id like to use a cantilever mount and nikon monarch scope with it.
anyhelp is aprecaited. i dont really want to go in any other direction.
iv tried a few mounts. i wont be drilling and tapping. only welding since its steel and if its jus right il leave it permanent

The problem is that this mounting system was designed/used in Russia beginning in about 1932 and was unique to this particular weapon using the mount that was designed for it, not "rings" as we know them today. I don't think anything has been fielded anywhere since, that would work in the fashion that you're looking for. However, you can (or, at least, could) get a repro of this base/mount that has 1" rings.. Check with John at Accumounts to see if he still has them.

As you can see in the pic, the "rings" are actually an entire mount that slides onto the base.

I went and measured. The PE mount is going to be about .933" or 23.7mm. The weaver rail is about .8" or 20.5mm. So, no, they are not going to fit.

No problem.

I'm more of a purist when it comes to vintage rifles. However, IF I were going to use a Mosin as a base for a more contemporary setup, I would opt for a more modern base/rail. One advantage is the availability of more mounting options/optics. Another would be that you can space the rings out further for a more stable platform. As you can see from the picture above, that short base/mount doesn't afford you a whole lot of room to work with!