Don't forget the K31's typically have a .304"-.306" groove diameter barrels for that slightly smaller 7.5 mm bullet. It's not normally too much to ask a rifle to swage down a .308" to fit, but that's still enough groove diameter difference that the freebore may be only .306"-.308", so your bullet is probably stopping on the mouth of the freebore and not in the throat of the rifling. Hence the big difference in COL.
If I had one of these rifles, I think I would moly-plate the bullets to ease the swaging process. I might then push them through a .304" sizing die so I could seat them out where they belong. Before that, I would slug the bore and the throat and freebore to see what I was actually dealing with.
Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Rifle Instructor
NRA Benefactor Member