I had e-mailed JD Jones at SSK and asked this very question in relation to a T/C Contender 300 Whisper barrel... Following was his reply!
Quote:
1.35O should work. T/Cs chambers are looser than ours. Not enough to make a load difference. Their twist rate is 1--10.
Making brass from 223 or 5.56 can be EXTREMELY dangerous. Our brass is made from GI or Commercial is made correctly and is safe. We have so for found about 40 headstamps that if made by the average handloader can be very dangerous. Most of the people on the internet are using counterfeits of varying chambers etc. It isn't a place to go for good information. Normally trimming back .010" is adequate. So far it has taken over 20 firings of 221 in my chamber it get uneven enough to warrant trimming. JD
Your best bet is to get a .30cal chamber length gauge from Sinclair. They are inexpensive and easy to use. Plus you can use them on any other .30cal rifles you have, just convert a piece of brass.(easy)
I found that my chamber allows WAY more brass length than most people trim theirs at. I personally think that it is important to have the brass to about .020 short of the chamber max length.
I will go look up the dimensions if somebody wants, but if I remember right I pretty much would never have to trim with converted .221 brass for sure.
My chamber is one of JD's.
FWIW, I use a Lee .221 FB case length gauge/cutter. I cut .223 at the shoulder, full length resize, and then chuck the brass in my drill and trim down 1.388"-1.390" with the cutter.