My new to me this weekend

BSA .375 sporter barrel length is 25". Muzzle diameter is .665". The following are dia starting from the muzzle towards the receiver. Hope this helps. Let me know if you need anything else. Best of luck on the build.

ML .665
1 - .667
2 - .668
3 - .669
4 - .669
5 - .672
6 - .672
7 - .680
8 - .680
9 - .682
10 - .688
11 - .694
12 - .702
13 - .710
14 - .725
15 - .739
16 - .757