In reality the TAR21 is not significantly lighter than the F2000 (the Mk20), yet in game it's 25% lighter. It's carbine in game is as light as the lightest SMG, and the same weight as the heavy pistols.

Likewise, in reality the M14 EBR (Mk18) is well under 1/2 the weight of the Cheytac Intervention (M320 LRR), but in game it's exactly the same.