When you kill a enemy your profile will save the body counts with that weapon.
After a while you are able to earn a Bronze star and so goes on. For these stars you get points for unlocking items.
Also when you kill a enemy with your weapon he gets information about you how great you are with the weapon.

Weapon Stars:	Vehicle Stars:

As you can see here I added a screenshot earning a Gold Star with the M24 Sniper Rifle.
The one you killed shows up a bar with all my information like:

YOUR KILLER: [NL] Underp4ntz
RANK: 12
HEALTH: 100%
KIT: RECON
WEAPON: M24
SPECIALIZATIONS: NONE

On the WEAPON tab you see a Gold Star with a number 4.
Thit means your killer has reached 4 Golden Stars with his M24 Sniper Rifle.