I think you have to keep in mind when I say SAW, I mean an M60 or M249. Under no circumstances is an MP5 a support weapon. A full length G36 might count, but that's stretching it at times. High caps on an assault rifle do not a SAW make.
I think your problem is simply rectified by playing with real capacity ammo rules. That should pretty much get rid of the box mag on the MP5.
What it sounds like to me is you're beginning to experience the division of milsim and skirmish gaming now that your local community is big enough. Some of you are going to want to play with a high degree of realism. The others are simply going to want to shoot things. There is no real solution for this problem other than specific games designed for the specific interest. Try holding a 'realcap' game and see what kind of attendance you get. You might find it much less than your average game, but you'll certainly get the kinds of people you want, and won't suffer from MP5 Boxmag-itis.
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