I have read that there is actually a dovetail,similar in size to what you find on a .22 rifle on a Mosin. At the gun show today,I saw this:
It was 10 dollars. Similar ones can be found for 10-20 dollars on a Google search. Mine was not in a box so I have no part number,but this link calls it a .22 Picatinny/Weaver adapter.
http://www.opticsdome.com/leapers-low-profile-airgun-22-to-picatinny-weaver-rail-adaptor.html?___store=default I dont recommend or not recommend this link,I have never done business with these people,I bought mine at a gun show. They list it for 7 dollars however,which seems like a pretty good price,but maybe you can find an even better price somewhere else. At least it has a description and part number.
The wheels started turning. A half hour later at a table,I saw an open reflex sight for 40 dollars. It was genericly packaged and called a "Tactical 4 Reticle Sight". I saw them at other tables sold under half a dozen brands and names,with prices ranging from 40 dollars to 100 dollars. They all appear to be the exact same sight. There apperantly are two models,one with a switch to go from red to green,the other just red. Mine has the switch. There are also slightly smaller ones,perhaps even smaller than a Fast Fire II and some that only have a dot reticle. This was cheapest they had there though,and of the others that were as cheap it seemed to be the one that worked the best for me. I dont know if it will break after a few shots or not,but my theory is,it will work long enough to see if the entire idea of a reflex sight on the Mosin is a good idea. If it breaks after a few clips of ammo,and it does not work,Ill try a long eye relief scope. If it breaks but works well before it does,Ill get something like a fast fire. Either way,I'm only out 40 dollars.
So,how to put it all together. First, I went here
http://7.62x54r.net/MosinID/MosinDisassembly.htm I removed the two barrel bands (actually,they would not clear the front sight,but that's OK,they didn't have to) and removed the hand guard. I then followed the directions for removing the rear sight by drifting out the two pins and then drifting it forward off the dovetail. Mine was as sharp as a knife. Be careful if your doing this to yours. No one bothered to deburr a part that would be covered by the rear sight I guess. (Just get those rifles out into the hands of soldiers to kill more Nazis!)
Now I just mounted up the Picatinny adapater.
And then installed the sight.
This thing is on there solid. I am not worried in the least that this mount will vibrate loose while firing. Best of all,it was only 10 dollars,but I would not want one of the ones that replace the rear sight blade even if someone gave me one of the 50 dollar ones for free. This thing isn't going anywhere.