FWIW, Manitoba is kinda considered to be "Milsim Central" and our LMG rules are rather lax for low/realcap restricted games. Machine gunners are the only ones allowed to carry hicaps and box mags as well as loose ammo, although if they're using hicaps they may only carry one so it can't be loaned to a rifle user.
Now that I think about it, we probably haven't had much of a problem with LMGs simply because we didn't have a lot of them around. This season there will be a LOT though, seems every man and his dog wants one now. Off the top of my head I know of over a half dozen, mostly CAs with at least one STAR. We'll probably introduce some new rules here and there but for the most part we like to assume that the player is going to play responsibly with their equipment.
I'm going to disagree with restricting box mags to 200 rounds though. It's generally accepted that it takes at least two BBs to accomplish the same task as one real bullet, and you usually can't get all of the BBs in a box mag to enter the feeding mechanism when it gets really low. If you're intent on restricting machine gunners I would recommend a load of 500 rounds in a box mag, otherwise most people with CA or TOP M249s will just opt for multiple under-filled 300 round hicaps since they're a lot easier to carry and swap out.