1biggun thanks for those barrel links - I never would have thought about Numrich! That's half what a PAC-NOR barrel runs, and I can obviously lathe it down to fit the profile!
Quick magazine update - we hand-cycled the longest factory loads we could find - 140 grain - which was 2.98" length. Below the max COL but still factory load. The MG13 magazine does *NOT* work for 6.5x55 in that it is too long by about 3/8" or so. That extra length is enough that when the rounds begin to ride forward in relation to the round below them, they tend to tip downward from the push of the bolt when the shoulder is pushed that far forward.
We ordered a pack of those IMA 7.5mm French mags for the French 1924/29 MG:
Modern Firearms - MAC M. 1924/29 light machine gun
According to my Hornady manual, that round measure just a little bit longer than the 6.5x55. I'm hoping that it will be close enough to keep the rounds from doing that slide-and-tip thing in the mag. So once they ship the mags I'll have more info.
Chamber pressures should be on the relatively low side of .308, as he doesn't load these incredibly hot. So I am inclined to say that the RPK parts will be adequate due to all the comparison of the Yugo .308 bolt vs. the 7.62x39 bolt (remember all that talk!?!?). Since this is relatively less pressure than the .308, I believe it should be fine. However I can't seem to find a readily available pressure table of the different loads that are available. He's running the 100 grain rounds - almost the smallest rounds for that cartridge.