What type mount it is ? Is it steel mount ? Photo ?
Generally, you should only use 1-piece side mount with Mosin and never use 2-piece mount where front and rear ring are not placed on the same 1-piece rail.

Stock is also very important with Mosin. I have seen some very accurate Mosins, but if you want to make it shoot you really have to know what you are doing. All things designed to improve more modern bolt actions do not work with Mosin.

Do not know what ammo you shot, what is your bore exact caliber and what is your bullet diameter ?