Hello:
I have a 38/55. 4 or 5 year old barrel. Took a lot of work to hone the 45/70 chamber to match the large barrel.
Somebody a few days ago posted a 38/55 with a .375-.377 barrel. I read H&R corrected the
barrel problem and that meaurement would varify that. 381.5 means you have an older barrel.
If you understand what I am saying you can see the chamber and the barrel are not comparible without some work.
I shoot now .380 cast bullets. I use a Lee mold because it is easy to hone to a larger diameter.
I have never slugged a barrel cause I just buy the different bullet sizers and which ever shoots the best is the barrel size. For the 38/55 I have sizers .373,375,377,378,379, and 380. .373,375 and .377 are for paper patch. I need to try .381 lubed bullet. I shoot both smokeless and BP.
I hope this helps.