I've thought about the same thing - a bolt-change caliber conversion
I've done some reading on this for some time and there are actually two 5.56 versions floating around out there. Problem is, as I recall, is that no "official" 5.56 caliber was created, only by aftermarket factories like Norinco. So watch it with the trunions.
As I recall, the Chinese 5.56 version uses a 7.62 sized bolt and trunion, and all parts except the bolt would interchange w/ any other Chinese 7.62 part.
THe other caliber is that from the Romanian / Euro factories and I think those are the ones that use the 5.45 bolt/trunion.
SOOOO... you couldn't use a Bulgy/Romy bolt in a Chinese rifle, and vise versa. I think there's even a 3rd pattern but don't remember at all.
I'm going to build one from the ground-up and use all Euro parts (probably from K-Var).
Again, I'm doing this from memory and never bookmarked those threads. Check the gunsnet Norinco forums. There's more about how magazines interchange than anything else, but there are some discussions about the bolts.
As far as the receiver, you could enlarge the rec to accomodate any mag, since the mag well dimples keep it in place. Don't see any problem with that.
hth,
- Jerry