My version of the NRA rules dates to Jan 1997, hence may be null and even void. Rule 3.1 Hunters Pistol indicates barrel length limited to 10 3/4 inches. Magnaporting or similar gas venting is allowed. Since this is integral with the barrel, it adds no length. Also only barrels from the same manufacturer are allowed.
I have been told, (haven't seen anything in print) that since T/C no longer makes 10 inchers, now 12 inchers are the maximum. Not allowing Bullberry and Fox Run barrels doesn't make a lot of sense, so maybe that's been changed too.
It is a great game. The guys I shoot with try not to get so caught up in rules that the fun leaks away.