After my earlier reply to this post last summer I purchased a Bushmaster 7.62x39 upper in November that I use with my M&P-15 (5.56) lower. It came with two 26 round magazines from Bushy - both made by Ammunition Storage Components aka ASC - and is sold as a complete upper assembly directly through Bushmasters web site, though I got mine at a local gun show for less than the MSRP of $799. It has a 16" 4150 chrome-moly steel w/chrome-lined bore/chamber with a 1 in 9 twist along with the usual AR upper extras (handle, sights, hider, dust cover, etc). I've put about 150 rds through the gun and have experienced no failures or feeding problems. From what I've read it's the magazine that is the weak link with the AR when it comes to firing the 7.62x39 round, but mine have worked fine. I ordered several more mags from BM soon after I got the upper but I'm on the waiting list since mags are at a premium these days.
I've cut back my shooting in the past few months due to ammo being both scarce and expensive since the holidays, but from my limited experience shooting the gun the Bushy upper is well made, tight fitting and able to shoot 2" groups at 100 yds with factory sights. I like it. It seems I now have the best of both worlds and if the price ever comes back down and availability goes back up, I may just get a second one... or maybe a 300 blackout!