I know this is an old thread, but regarding rechambering the M1 Carbine in 5.56 NATO, 7.62x39mm and 7.62 NATO, Ingram did exactly that sometime in the 1970s.
They use AR mags, AK mags and FAL mags, respectively.
Might need to source .30 carbine before bringing in more M1 carbines.
It aint cheap, It aint easy to get.
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7.62 NATO and 5.56 in an M1? Pretty hot stuff!
Now .300aac would be a small but nice upgrade.
I last saw m1 ammo at cabelas for just north of $30/50. I began reloading after a few boxes of that. Actually if you consider the cost of new brass a case could be made where its just as economical to buy the factory loads for the brass. Once you deduct the worth of the case the powder n ball in there is pretty cheap.