I am building an ar pistol in 769x39, has anyone ever built one and were their any issues that came up. I have read that mags are one issue.

I had a 7.5" uppe is 7.62x39 for a while and never had issues. I only used C-products or ASC mags and they ran 100%. The only other oddity is to make sure you have the "enhanced" firing pin - its slightly longer to account for the recessed primers in the 7.62

^^^. This currently have a 7.5 upper pistol. I find the 10 rnd asc mags work better. I will be getting a couple 30 rndrs and check them out.