
I'm of Two Minds

Don't bring me into this

I think I have mentioned inna past, I don't like Uberti. Beyond that, the bullets being used may well not be optimized for a .375 barrel. But before tossing the whole thing in the "Big Green Safe" out back, I'd first try a full diameter spacer thingie at the end of the Arbor. A nice Brass washer, filed to fit, coupled with a nice snug Wedgeie and remembering to hold yer tongue right.
Along with the above, I'd quit messing with the odd cast bullets and go with Swaged Full Hollow Base Wadcutters. I've had great result with them in my guns with .375 bores. No guarantee with your gun of course, after all, it is a Uberti (Yech) . And, with full Wadcutters, you should be able to run 38 Special cases and skip those itsy bittsy teeny tiny periody correct powder buckets. Just a thought. Or three.