I've been using King Arms 68 round metal mags in my CA M15, JG HK416 and custom C8, and they're always worked without issue. With basic maintenance (occasional light lubrication), they alway feel reliably right to the last 2-3 BBs, which is normal for King Arms. Their follower protrudes farther out of the mag than most other brands, so you only end up with 2-3 BBs that don't feed instead of the 5-6 with other mags. And they're quite affordable too.
If you want cooler looking mags, then definitely the Magpul ones are the way to go, but they also cost more. So it all depends on how much you're willing to spend and the look you're going for.
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