Hi, guys,
If you are talking conversion of the old cylinder, the problem would not be only a matter of pressure. The 7.62 x 25 is enough bigger at the base that the web between the chambers would be reduced significantly. Also, the Nagant chamber is bigger at the front end than the auto pistol cartridge needs (the chamber will easily swallow the whole neck of the case), so there would be a lot of case swelling and bullet upset in the cylinder, not a good combination.
Use of a cartridge like the .32 S&W Long in a new cylinder, while it won't take advantage of the Nagant barrel-cylinder seal, will at least not present these problems.
Jim