I have two Ruger American Ranch rifles, one in 5.56 and one in 7.62x39. The stanag mag well was for the 7.62x39 so that I could use 10rd Lar mags for 7.62x39. I do use the ACIS 5.56 mags for the other rifle. If there was a 10rd ACIS 7.62x39 mag I'd likely do that.