What means "mobility" stat of the weapon?

The effect it has on your movement speed. Heavier weapons = lower mobility = lower speed.

It means your movement speed. The in-game bars are only rough guidelines and are in most cases wrong.

with "1" meaning base move speed then..

SCAR : 0.98
SCARAB : 0.98
GRENDEL : 0.97
FELINE : 1
K-Volt : 1
MK60 : 0.96
L-Tag : 0.95
Jackal : 0.99
Marshall : 0.99
DSG-1 : 0.97
Gauss : 0.96
HMG Rip off : 0.60
All side arms : 1

So what does this mean? Well it means you move 2% slower with a SCAR than you do with a Feline, etc. The impairment that attachments give, if any, is still a mystery. Its been proven that underbarrel attachments give no movement penalty at all so far.

The impact on the movement speed is far to low imo -.- HMG Rip off being the obvious exception.
I think they could almost double all the penatlies. Or at least give sidearms a boost to 1.05 or something

ty i got it
few procentages is not so big penalty i think
100 or 97% of character move speed is not matter really

HMG Rip off : 0.60
That is actually 0.10. Just as I see.