I know, replying to myself looks strange, but I wanted to redirect to my last post...
After some research, speaking to others, and crunching numbers... I purchased a quantity of .45 Auto 230 grain Federal HST (P45HST2) for my Detonics Combat Master .45 Auto with 3.5 inch barrel. According to a very reliable source, this load still expands reliably from shorter barrels, even as short as 3.5 inches. I intend to water test this HST load in my Detonics.
Of interest, I expect, here are some of the numbers I got from my research/number crunching (P45HST2):
Federal HST average velocity from 5" barrel is 895.5 fps
Federal HST average velocity from 3.5" barrel (calculated estimate) should be 819.75 fps
3.5" barrel velocity is estimated to be about 91.5% of 5" barrel velocity
Based on all of these "calculated estimates", I only expect an 8.5% velocity drop when firing P45HST2 from a 3.5" barrel instead of a 5" barrel.
I intend to test this by firing this load from my 3.5" Detonics and also a 5" 1911.