Measured a few more times - looks like 378-379 and .395
Do believe this is a 38 caliber rifle . . . . Will do a slug and take it to somebody with proper measuring tools - a micrometer isn't good enough. This is to be expected, an oddball size. ;-) I had a feeling something was amiss when I took a 38 cal (357) Hollow base wadcutter and it rattled around in there. Heard of people using them in 36 cal rifles, hadn't planned on using them, but curiosity got the better of me so out one came and it just rattled too much. Plus, I can't see how a HBWC could work as where would the grease grooves go?