the American Ranch Rifle uses AR mags in 5.56 and 300BLK. The Ranch is the shorter barrel. The Short Action Ruger American, the longer barrel, uses an AI style mag, if I read it right. The above rifle is their Scout model and I think it uses AI mags too.
I have the Ranch rifle in 7.62X39 and it is super accurate. I took it to the range along with several Model 77's and a couple of # 1's and it out did them all.