I have not removed my barrel so cannot comment on the threads, but I assume they are small-ring Mauser threads - ie: models 94/96.

I shoot my M71/76 with weak action .45/70-type loads using moderate powders, ie: H4895 and bullets to 500gr. I re-chambered it for the .450 Alaskan (due to it's .457" groove diameter) to enable me to obtain useable brass at that time.