you can make a 7.62x39 conversion by just a 7.62x39 barrel and opening up a 5.56 bolt. and maybe a little trim job to get the firing pin to strike deeper.
so you could convert a 5.56 rifle for the cost of a barrel and a bolt. SMG may have some of those 7.62x39 barrels in stock. last time i looked they had set up barrels with the FSB/gas block barrel nut and so on for about $150--discount codes and club members discounts --you could get it for even less.
5.56 bolt were easy to come across for $40 before the panic. i have bought some unfinished 5.56 bolts for as little as $20! since the panic bolts have become hard to come by-- i did buy one from JSE for $40 not too long ago but thir still iffy for being in stock.
the only thing about a opened up 5.56 bolt as no one knows how long it will last. i got quite a few rounds out of the one i converted --and it ain't broke "yet"--LOL
don't forget you'll need 7.62x39 mags if you going to load up more than a few rounds and thir more pricy than the 5.56 mags and their not "fully developed" yet.
my 7.62x39 AR-15 mags are about half reliable with a full load of 30 rounds and seem to work best when short loaded.
a 300 blackout is even less trouble it basically is a 5.56 round with a .30 bullet . all you need with it is a barrel --and some ammo. of course the down side is you have to find ammo somewhere. i thought about rolling my on 300BOs --but i need these /308 bullets for 308s and other .30 stuff--