When I started LBT 27 years ago, all revolvers from all makers had tolerances as bad and worse than yours. Don't expect Ruger or any other manufacturer to come running with a new cylinder, or barrel or whatever because of poor tolerances.
Small cast bullets if fired with enough pressure to make them work at all (prevent leading) are going to swell to fit the chamber/throats of your revolver, so fitting them to the right size to start with certainly isn't going to increase pressure at all.
Fit bullets to the throats, make them at least 20 bhn, be sure your barrel measurement isn't just at the forcing cone, with the barrel larger at the muzzle, and it will perform perfectly. If the barrel is constricted at the barrel attachment threads, or rough at all, fire lapping it using the LBT lap kit will be mandatory to obtain optimum accuracy.
When set up properly, using LBT bullets of 280 gr plus if you choose the WFN profile, or 300 plus if the LFN profile, expect groups close to an inch at 25 yards, with about any powder charge you wish to shoot. Tuning the powder charge a bit will bring it down tighter, IF YOU ARE REAL GOOD, and an inch at 100 yards WITH THE GUN YOU HAVE is not out of the question. When you get the gun/ bullet set up right, it will teach you how to shoot a revolver, because you'll quickly learn that the bullets are hitting where the sights were when you pulled it off, and learn to correct your own errors. Buckshot groups like you are now getting will never teach you anything.
I strongly suspect that you have a rough barrel, tight throat problem as described above. The Montana bullets you have are close enough to the right size that it should should under 2 inches at 25 yards with about any stout load, if the barrel were right.
