Ok so when you go in to customization in the gun stats and press triangle for unlock progression it shows the attachment label right. For me tonite i got fifty kills on the marshal and should've gotten the unlock, but i didnt. Then it shows it again for a 100 WTF has anyone got this or is it my imagination. Oh im not a noob on these forums just to tell you.

If i'm not mistaken you get the first weaponattachment unlock at 25 kills and the second one at 100 kills (no attachement unlock for 50kills, just xp)