You have to get a certain amount of kills while using FMJ (Full Metal Jacket)
extended mags or akimbo attachments is the best attachments for me but everyone has there own play style so try them all.
The best class you can have is the following: SCAR-H FMJ or Extended Mags SPAS-12 FMJ or Extended Mags marathon stopping power commando This class does alot of damage and it can be used to rush enemy spawn areas or to rush objective
extended mags are and attachment for your gun when you get 40 bullit penetration kills with fmj
Penetration on mw2 means you have to shoot your enemy through the wall if you want extended mags on a sniper you need to do this on your sniper and the best attachment is a fmj bullet hope you get them extended mags. A good way to do it is to get someone in your party then go to barebones pro and boost from there get your friend to plant a tatical and ten shooot though a wall some times people will kill you but its bettert then having to get it by killing on other modes.
F2000 Thermal + Heartbeat Sencor, AA12 Silencer + Extended Mags Perks: Bling Pro Hardline Pro Scrambler Pro
Extended Mags
yes
get 30 FMJ Kills
it gives you more bullets
30 in Standard mags 40 in Extended mags 75 in Drum mags those were the common mags, there are also custom 60 round mags that look like 2 standard ones back to back that form a half-circle shape
Put on fmj and shoot through walls to kill people u need 40 of them
you can get extended mags with 400 kills with it