M21 Semi-automatic long range sniper rifle.
Unlocked at level 7.
This is closely related to the M14 assault rifle except, obviously, for the scope as well as a smaller clip size of ten. It also has around the same power as the M14 and more importantly the same recoil, which on a sniper rifle is extremely low. Multiple shots are easy to rattle off and stay on target allowing either a second shot or straffing across targets. This is useful as the power of the rifle usually requires a second shot to take down targets, especially juggernauts. It is also worth noting that the kick of the rifle seems to be exaggerated if you hold your breath whilst aiming. Therefore if you are planning to fire several consecutive shots just fire them without holding your breath and you will notice less pronounced and more predictable recoil.
It seems to be quite a forgiving and I found myself using it after I had unlocked other sniper rifles just because I liked its predictability. Used with iron lungs, stopping power and aiming for headshots this can be an effective rifle.
Due to the low recoil it is worth thinking about mobile sniping with the M21. However keep in mind that the M14 can be equiped with the ACOG and has a larger clip and you really need to get those head shots or have a fast finger to take down targets quickly.