AR 15 in 7.62x39

[highlander]

Need some insight if I could. I'm thinking of getting an upper for one of my ARs in7.62x39 now I know there are no hi caps except for the "Frankenmags" available and I can live with the 10 rounders from C products. Has anyone here any experience with an AR in above caliber?
The other choice is 300 fireball/whisper. The only problem is finding 221 fireball brass though I could use 17 fireball brass in a pinch.

 

[Coyote33]

How about .17 Fireball?

 

[Remsport Mfg]

I have tried to build a 7.62 x 39 AR that is reliable and have not been able to. All the mags out there except the old original Colt 7.62x39 mags and they are hard to find and expensive do not work correctly . The mags available all have nose down problems 5 rounds loaded in the mag and cause mis-feeds.