Next question.

It is currently a 410 which is just over 10mm, but there is more than enough meat in the barrel to take it right out to 16mm. However want to keep things simple as possible.

The 410 chamber is 11.9mm, with a rim of 13.6mm , so thinking to ream to 12mm, or out to 14mm. I can have the barrel profiled to pretty much what I want so thinking of c 100mm at 14mm od and then stepping down to 10.4 - ie 410 bore daiameter. That will give plenty of strength around chamber, but would I be better just teaming whole barrel to 12mm.

Should I drill or use an expanding reamer.