I have been thinking of buying one. what has been holding me back were the reports of feeding problems with the tapered x39 case and the necessity of using curved magazines. do I have to get a special lower receiver to take those mags or has the problem been solved and I can just slap the upper on a 5.56 lower? also, I have not seen any 7.62x39 AR 15 mags for sale. is there such a thing?