Interesting thread, glad I found it.
Dave, I haven't tried 375 ammo in the 38-55 CB since I hardly ever use factory stuff. But I can offer the following data fired in a CB with .3809" grooves.
All loads use Benchmark:
Horn 220FP 375"/37 gr =2001 f/s
Barn 255FP 375"/35 gr =1847
Barn 255FP 377"/35 gr =1892
CP260LFNGC 380"/36 gr =1989
The low sigma and small group with the Horn indicates that they may be slugging up somewhat.
The 377 Barnes grouped much better than the 375 version though the Cast Performance is the overall winner by a long shot. Pardon the pun.
As you discovered, OAL is best kept well controlled.
SOS
And yes, I still have all my fingers, and the cases extract effortlessly, though I consider the above loads max in my rifle.
