I’m sure a company could make funny looking large capacity magazines. It has a lot to do with the curvature of a magazine and size of bullets, I think. The larger the magazine the more it doesn’t make sense and the functionality will be questionable. Personally, I have a Promag magazine that holds 30 rounds. I live in California, but for about a week these magazine were legal to buy here. I also have some 20 rounders from Magpul. Magpul does have a 50 round drum available.

I’m not too sure what the question is. A capacity of a magazine is limited to whoever makes the highest capacity for that round. Not all 308 Winchester rifles are the same which is why some only have a certain capacity.

Monty, What is your question? There are standard capacity magazines, high capacity magazines, and drum magazines for almost any of the rifle calibers you talk about. The 300BLK popularity has nothing to do with using 5.56/.223 magazines. It is because of the round. There are high capacity magazines for sporting rifles chambered in .308. You are only limited by the size of magazine your firearm will except. The Tavor X95 takes AR15 style magazines, and while most of them will work in the gun, there are differing degrees of compatibility.