There are 100 round magazines available for rifles other than the AR15, but I would personally not use a magazine over 30 rounds. If I wanted dependability probably 20 rounds or less. Because of the weight, 10 rounds is good in a .308.
Commercial .223 ammo is sold in 20 rd boxes. I like a reliable 20 rd mag with a good follower. There are some unreliable followers that jam easily. The 30 rd is even more susceptible to a jam. A cheap magazine is no bargain.