just get kills with the gun, then when you unlock attatchment like the red dot sight, getting kills with that attachment on will unlock further thing like so:
grenade launcher - underslung shotgun
silencer - heartbeat sensor
red dot sight - holographic sight
FMJ - extended mags
ACOG scope - thermal scope
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bling pro allows for your secondary weapon to also have 2 attachments
One of the best secondary weapons in CoDMW2 is the 1887 shotgun. I recommend attaching akimbo to this for multiple damage. You must be quite a high level to unlock this weapon. Also it is good if you have FMJ and akinbo if you have bling pro unlocked!
get 50 silenced weapon kills after you prestige
It depends on which weapon but for SMG's you need to unlock "rapid fire" and get 30 kills with it attached to get an akimbo SMG. For some shotguns, you need 15 kills to get akimbo. For handguns, 50 kills are needed. For machine pistols, you need 75 kills.
on ps3: triangle; the regular weapon swap