I have not slugged the barrel but did query Uberti customer service and they have stated it is a .312 bore. So far, I have been unable to find any .312 bullets so my only option has been .311 and the only ones I could find in 174 and 180 gr. has been .311 Hornady FMJ/BTSP and PPU lead core spitzers. I was able to buy some PPU 150's in .311 and will be trying them out as well. If anyone knows of a cast bullet supplier that has gas check .314 I'd like to check those too.
It would appear the 303 Brit has suffered in the priorities of the manufacturers. Even finding .311 has been a search with most options shown online have "out of stock" and "no back order". My recent visit to Cabela's found empty shelves and I have found that primers are in the same lack of availability. Luckily, I have a half brick of Winchester and a couple 100's of S&B and Federal.
I haven't checked the twist yet but took Uberti customer service at their word that 174's and 180's were first choice.
As I say, I will be removing the forearm and having a look see and see what may be causing the problem. If I find anything, I'll post it.
