I noticed on Windham weaponry they have a Conversion kit for the 7.62x39 with everything but the upper. If I ordered that could I get just the lower, maybe from MGI directly, their site is still up but I know WW bought them. Or could I use these parts on an upper that gets milled out for clearance with AK47 mags? My hope is to put a Rare Breed FRT in the modular lower and be able to use AK mags or AR mags. My dream is to build an 18” 7.62x39 heavy upper and blast some steel drums through it.

I doubt the FRT has enough spring power to reliably ignite 7.62x39.

You could do it with the Fostech Echo and their special 7.62x39 hammer spring.

I've suspected that WW bought them but I didn't see that 'officially' anywhere. Do you know that for sure?  Makes me wonder why MGI just doesn't take their site down.  I've had a couple friends reach out to WW and MGI several months ago.  All were told that they couldn't get anyone from MGI to respond and that WW said they would only sell a complete gun not just the lower.  When you previously could just get the stripped lower for $99 from MGI.

BTW, I just use an 'enhanced' firing pin with my 7.62x39 and 5.45x39 configurations and a standard power spring just fine.  I run them both in full auto.  
http://www.c3junkie.com/?page_id=185
http://www.c3junkie.com/?page_id=171

Amphibian, that video is exactly what I am looking to achieve but 18” barrel. Love that build, looks like a blast. As for the first post, meant to say WW bought the rights for MCS.

Also, I must say, I am a huge fan of your gas block design and have managed to hunt down 4 of them to use on my favorite builds. I’ve shot hundreds if not thousands of rounds through different builds I’ve put together using them.