Good stuff Ron :)

any consensus on whether it is relatively safe to assume a GM barrel not marked otherwise in a T/C Hawken is 1:70..?

it's a possible trade/swap and distance precludes actual checking; the owner is nice but clueless.

I don't think there is MP. I guess it matters when it was made. So unless the barrel is marked LRH the only real way to tell is looking down the bore or running a tight patch down and measuring the revolution of the rod. It may not be exact but will be good enough for an educated guess.