lots of info still needed to answer that one, but here goes; (all of this is a wild a$$ guess).
In reading lots of articles over the years, this is based on metallic cartridges, and actual tests done over the years by notable gun writers
(Bob Milek, Wiley Clapp, Mike Venturino, etc...) yet so many variables apply, and every case is going to be different, that said;
average velocity loss is about 25 fps/per inch cut, so a 24" barrel cut to 20" should lose about 100 fps, a 29" barrel cut to 23" should lose about 150 fps.
There is no set rule for these results, Magnum cartridges may be less susceptible due in part to their higher start speeds.
pistols and revolvers are even less susceptible due to their shorter starting barrel lengths.
Bob Milek did his testing with single shot Contender pistols starting with 14" barrels that used bottleneck cartridges,
can't recall for certain but I'm thinking 22-250, 243 Win , 7mm-08 were the cartridges. It's been a long time and the memory fails me.
Anyway, hope that helps?