that's about it.Hmmm, wonder how it would feed from ar mags. I will have to check it out. I assume you meed the magwell part that separate from the main stock? Forgive me for now knowing the exact terminology.
I can't say positively that it would work, but I also think it's very unlikely it wouldn't.
Now the mags themselves may need some attention, at least the larger ones. I've not yet had an x39 AR so I don't know which mags work.