C-Products Dura-Mag stainless steel works with most 7.62x39mm "Fireball" uppers that have the double-wide feed ramp in the barrel extension explicitly for 7.62x39mm ammunition (this is different from M4 feed ramps). back in the day when Colt released the Sporter Lightweight in 7.62x39, the majority of feed issues were not using an altered feed ramp (which allowed cartridges that weren't being presented perfectly to jam into the flat in the barrel extension) and the use of a 30 round adapted magazine which was a pile of crap. it worked quite well with the 20 and 25 round straight magazines though.
over the years (Olympic, Bushmaster, Model 1 Sales, even late J&T models) came out in 7.62x39 but were wonky for varying reasons. it wasn't until C-Products and Bushmaster collaborated on the XM-15E2S 762 rifle (sold through Model 1 Sales and even Del-ton in the late 90's) that the solution was learned:
1. a single double-wide feed ramp of a precise geometry in the barrel extension
1a. note the ramp geometry is dished at the sides and "double-width".
1b.
2. new feed lip geometry for an angled presentation to the bolt (C-Products Dura-Mag SS 7.62, PN# CPD-762X39-MAG)
3. an altered bolt that has a widened FP channel and used an altered hardening QPQ process (early M1S, Colt, LWRC, OA, et c had cracked lugs because of this - the new business is a Y/M 9310 bolt from The Buffalo out in Ventura, although if you have a gentle gas system like a 16-inch barrel and a gas bleed on a very wide gas port you can use a Bushmaster bolt reliably).
4. an enlarged gas port (0.083" iirc, but i haven't taken one apart in a long time) to lower overall gas pressure to more typical AR-15 levels. this also has the effect of lowering bolt carrier group velocity. note that this is for a 16" barrel with carbine-length gas system.
5. go with a different solution entirely: throw all the above away and buy a purpose built rifle (KAC-47, CMMG Mutant, Colt Model CK901, et c).
those are the options. these days if someone wants me to build (rather than buy) a rifle for them, i'll usually go with a Buffalo bolt, mostly quality OEM ar-15 parts with a couple premium options like LMT, and a Bushmaster or DPMS (Wilson) barrel.