Interesting, thank you for that Gren. My regulating load measures 1070 fps at about 4' if I remember correctly. Speed of sound at sea level is 1125, so I'm still subsonic at the muzzle? I had the load pressure tested and produced 1,185 fps out of the full choke tight .727 pressure barrel. My bore is .736 and slugs are sized to abt .733, so a little more room to mellow things out.
Kind of fun putting all the pieces carefully together, then realizing the exact reason a certain combination of wad/hull/powder/slug/alloy/crimp/lube/etc. works is still a mystery.

15:1 alloy was my second try. I started with 20:1, which did not engage the rifled choke sufficiently. I looked into the 2% Antimony, but did not look like something the DIY guy could alloy correctly.

I now think about these slugs moving through a Fosbery choke or tapered oval bore as something between fluid motion and an extrusion process. The changes that the slug undergoes as it is forced into the Fosbery choke is massive. The choke swages down and elongates the slug and the rifling leaves 7 distinct facets on the base of the slug. The projectile shape is really made in the choke. It is really a very different process than firing a slug in any other conventionally rifled barrel.

I'd love to get a batch of 870 (or similar) barrels made up with Fosbery chokes. Would be fun to experiment around with. Maybe we should try to get a Nitroexpress group buy list together? Anyone tooled up for the Fosbery choke?