Absolutely they can check magazines. Spot checks work well as well. If they know you're randomly looking, it'll discourage players from breaking the rules. We only half load our mags, because they come in 120 round mags. Our philosophy is two BBs per bullet, but we'd follow any rules at any game we attended. More competative play isn't a bad thing. It's usually fairly easy to pin magazines you already own.
Basic rule of thumb is based off of M4/M16 as they are vastly most common.
High: 300 (or anything above mid)
There are box and barrel magazines as well, that can hold thousands.
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