Noreaster, which state are you in? In NH it only has to be limited to 5 if it's a detachable magazine. A 6-shot revolver is good-to-go (as-is a .30-30 with 6 in the tube).
Anyway, to the OP, as was said, it's a size issue. The L-frame and GP-100 were designed around the .357 and don't have a lot of room for "caliber improvement." So the full N-frames and big Redhawks and Super Blackhawks we get.
NRA Master, Highpower Rifle, Across-the-Course
NRA Expert, Highpower Rifle, Mid-Range Prone