I found this ...
http://www.uspsa.org/rules/2010MultiGunRules.pdf ... check out appendix A4 ... says minimum caliber .308 and minimum 320pf ... under "
multi-gun" ... in another column, for minimum caliber, it says "
no" ... possibly a misprint ... who knows.
No clue if anyone's following that, to the letter. Sounds like they can do whatever they want. So I reckon ya'd hafta go on a match-by-match basis.
That said, I'd be very surprised if anything .308+ dia. and 320pf+ would not be considered "
heavy metal" by anyone.
Looks like a 123gr 7.62x39 would hafta go over 2,600fps. You can do that, I reckon that's "
heavy metal". I wouldn't think that'd be possible ... but I really don't know, fer sure.
No clue what other cartridge ya'd care to try, to meet that requirement. Probly cost more in brass/ammo, at the very least.