Shooting 22 inch GM 1x28 Flint barrel
 

Took my GM 22 1x28 put it on My Lyman Deerstalker. I shoot 80 gr of 2 f black with a home cast 300 gr 430 bullet shoots very well. I also shoot 70 gr 2 f with a 400 gr 458 resized to 452. It is basically a 45/70 carbine shooting at 50 yards of bench 3 inch group w/ iron sights. The Lyman came with a 1x48 54 cal.

I have the same barrel and mine shoots the same .430 240 grain XTP with 80 grains of Goex 2f very well. It also shoots the 300 grain with the same powder charge. I have even shot some 375 grain Buffalo Bullet SSBs out of it with 60 grains of powder and it does pretty good. I get about the same accuracy as you are describing. 3 inches with the open sights at 50 yards. Makes for a great little walking around rifle.