I have a 32/20 barrel with the same problem. With pretty much any bullet, if i try to seat it to the cannelure, it is too long to chamber.
I took my brass, trimmed it down .15 of an inch, and ended up with some short funny looking cases, but they work fine, and no more problems. Since the case neck is pretty long on a 30/30, you might give that a try..TC would probably rethroat the chamber fror free if you explain the problem, especially if it wont chamber factory ammo, but if you dont want to have to take the time to send it in, just trim the brass a bit and see if shaving a few thousandths off might help...