If it is shooting low, the easier fix is to simply shorten the front site post. This will give you a couple of hours good shooting fun. Consider yourself lucky. Many of the Mosins shoot a bit high. To correct that, the front post must be lengthened or replaced with a longer one. The post on the Mosin is a little more than a mild steel wire. To replace it, the front sight must be removed. In your case, you can adjust your site to match the POI by merely shortening the post. Go slow and have fun.