As a tube is stretched, the diameter is reduced. Just heat the 270 barrel till its glowing, then use a winch or come-alongs to stretch the barrel out a few inches. Stuff a caliper in there and stop when it hits .243.
I think you're on the verge of a break-through; with a slight problem. Stretching the barrel will slow down the rifling twist, perhaps to an extent that could effect stability. Fear not, there is a solution for that. While the barrel is red-hot simply slap a pipe-wrench on the muzzle and twist. This opens up the possibility of both a tapered bore and gain twist rifling. There is the risk of unthreading the barrel which could be minimized by hammering the receiver ring flat before starting.






















































