You might want to research the 7.62x39 in an AR. I've heard it is not the best choice of cartridge for it. The mags may not feed as reliably as 5.56, 6.8SPC or other similar cartridges. Plus the bolt face has to be ground out to almost nothing which may also contribute to feeding and extractor problems. If you go with 7.62 be sure to copy exactly what was used by someone with zero issues: brand of bolt, mags, extractor, etc.
The 7.62 should be a good deer cartridge but probably one of the worst for the AR. The SKS and AK were designed for the 7.62z39 cartridge, the AR was designed for the slimmer rim and and straight side case of the 5.56. There are 6mm, 6.5 and 7mm variants of the 5.56 case and of course the 6.8 and the .300 Blackout (which happens to about replicate the 7.62x39 ballistics with a 125 gr bullet). You might consider the .300 BLK instead of the 7.62x39. The .300 is designed for the AR, replicates the 7.62x39 ballistics at 125 gr but also is designed to handle bullets up to 200+ gr for suppressor.