The weights of all the various assault rifle magazines differs greatly, the M4A1 magazine (30rnd stanag) for example weighs "under 0.25kg" while the KA-101 magazine (same cartridge and capacity) weighs "just under 1kg", which is the same weight as the KA-M magazine weight (7.62x39).
The LAR magazine weighs "just under 0.5kg" by comparison, which makes little sense.

Current weights:

• KA-101: ~1kg
• KA-74: ~0.5kg
• KA-M: ~1kg
• M4A1: ~0.25kg
• LAR: 0.5kg

In reality a standard stanag (aluminium) 556 magazine weighs around 0.5kg fully loaded, I'd imagine the polymer 556 AK or 545 magazines would weigh slightly less with the AKM magazine weighing more (around 0.8kg).
The FAL magazine weighs abouts 0.7kg.

All of these weights are fully loaded!

Something similar is also going on with the SMG magazines, UMP-45 mag for example weighs around 1kg....
But I don't have these weights available atm.