The Beretta AR-70 was hands down the most 'comfortable' 5.56 weapon I have ever owned. Excellent ergonomics and it shouldered vey easily. The lack of available spare mags really soured it for me. It came with one factory mag in the box, and I tried for over a year to get additional mags. Not being able to have more than one mag pretty much made it useless as far as I was concerned.
I had the very same issue with my Galil ARM. It came with one 35 round mag and I managed to obtain a 50 rounder from the dealer. That was it. It was big, heavy and pretty much worthless for holding off the hoards of invading Russians that we were expecting in the early 80's..