30-round mags exist on the commercial market, and some of them work very well. The M-14 is a great rifle - accurate and comfortable to shoot. And it's .308, which rocks.
Later, on I&I duty, we had some M-14's in our small armory. We took 'em out and had a blast shooting them.
I agree they're great weapons. I was just commenting that they are already at the limit of what I'd want to carry with a full 20-round magazine. A full 30-rounder inserted might be heavier than I would want. Just give me plenty of 20-rounders.
I'm in the process of selling some acreage, and when that happens, I might go buy some new gear, including a new Socom II carbine.