Jim:
With a little surfing I find twists ranging from recent productions of 1:18 to about 1:22 for more ancient units. Seems all should be capable of stabilizing the typical 405 slug to at least 3-400 yds. In theory one should be good to go, as that's the slug they were built to handle.
I've used cast 405's in a 45-90 Sharps and they're flying true to at least 100 yds with BP. I've not stretched that out and will be going heavier (545 gr) when I do. Velocity is probably around 1,300 to 1,400 fps with the first firing of those cartridges. Might be able to speed that up in the second loading being able to fit more powder when the cartridges are up to chamber dimensions.
Any idea what velocity one is getting with your load?
My Guide Gun is fun to shoot with the typical factory fodder going out at about 1,250 fps also with a 1:20 twist. Same for my friend's unit with his cast slugs. I jacked that up a bit with some hand loads to around 1,750, and it kills on both ends again though with jacketed bullets. It is not pleasant to shoot.
About my only suggestion is that one mic a few slugs to see what they measure. Also, if one has a BPCR neighbor about with a lube/sizer, one could try those slugs with a gas check installed. The examples I saw with my surfing show a beveled base suggesting there's room for a gas check there. Could be there's enough heat/pressure to cut past the belts, and that would create all kinds of issues.
Gas check linkMight also try them in some other rifle to see if results are similar.
One could also quiz the Shiloh forum and likely get an authoritative answer. First though, one will have to take a beating from that crowd for using smokeless.
Do keep us up on your findings and good luck.
