Re: barrel length vs case capasity
If you like, you can go by expansion ratio, compare your case capacity to other common rounds' ER.
But I'd say that case would know what to do with 30" of barrel, but the longer you go, the more harmonics are involved (barrel whip) which is mitigated by a heavy barrel or possibly BOSS-type contraption.
Strictly speaking about powder burn, I think you'd need to get into the 40" or longer before you'd actually start losing velocity.
To give you a comparison, I think I read the 22LR needs something like 16" of barrel before you start introducing more friction than the powder burn will overcome, and that little case holds something like 6g!
[edited to add:]
Get the longest barrel you can stand. It's easy to cut back if you're not happy.
[ 02-01-2004: Message edited by: youngun ]