Apparently a magazine's weight is not affected by the number of bullets in the magazine. Any spot in the code where the magazine weight is used for burden calculations or displayed ought to be something like this (I have no idea what the actual variables are that Arma 3 code uses):

MagRoundsRemaining / MagRoundsCountMax * MagWeight = The Used Mag Weight when playing & running around.

I realize empty magazines have their own weight, but something like this would be a good start.

For that matter, I'm seeing most mags in Arma 3 being roughly half the weight they should be. So, I guess we also need an Ammo Weight X2 check box in options & settings.

The two could even be combined: MagRoundsRemaining / MagRoundsCountMax * MagWeight + MagWeight = The Used Mag Weight when playing & running around.

I think I prefer that last suggestion to the two separate tweaks.