If the scope is working properly and the mounts and rings are solid, the problem is probably the bullet or charge. Try another bullet. With a 50 cal ML, I start at 90 grains. I go up to 100 if the groups are still tight.

Right now I'm shooting 100 grains of loose pyrodex and 350 gr Hornady FPBs. 100 yd shots on deer are not a problem.