After launching several hundred 260g LWNPB slugs with my 410 +P+ Marlin lever gun i am finally getting decent hunting accuracy. What finally became clear to me is reloading this 410 like it is cast boolit rifle does NOT work. The faster burning powders like SR4759 and I4198 fail because of the long forcing cone. The boolit (slug) in a roll crimped hull must travel 0.7" before engaging the rifling. The faster the powder the more velocity the boolit had when it engaged the rifling and tries to start spinning. My proof of this is watching groups become patterns as i increased the powder charge.
Couple weeks ago i switched to medium burn rate powders and accuracy immediately improved. I now have one load that will shoot less than 2" @ 100 yards. A lot more testing and load workup is needed before i hunt with this 410, but i can see the light at the end of the tunnel. I just hope the light isn't on the front of Casey Jones locomotive.
I have a target pic but i wrote the load data on it and i ain't smart enough to redline a photo.
BB