If you're talking about cutting back a factory 308 barrel, it ain't gonna work Skippy. On factory barrels the taper starts right where the threads end, so if you cut off say 1/2" from the breech to rechamber you'll have to extend the threads and the barrel won't be large enough diameter for them.

I thought someone was offering 7.62x39 prefits at one time. Eabco or PacNor maybe?

Getting it to feed/eject is another matter all together. Have fun with that!