Loaded OAL is an issue. I loaded some true Keith bullets in my .41 Marlin, and they were just a hair too long to cycle properly... vs the standard commercial psudo-Keith bullets that work 100% I also use some heavy Cast Performance bullets... with a wide, flat nose... they also cycle 100% so it's not necessarily the bullet nose profile, but they are shorter than even my standard 215grn SWC bullets I load.
There are also some ways to clean up the Marlin to feed better.