The AS50 appears in multiplayer, being unlocked at level 22. It has extremely high power(it is par with the Barrett) whilst it can kill from the abdomen and up. It is suggested to "camp" or provide sniper cover to your team since this rifle is not fit for a situation where you're against a Sub-Machine Gun or an Assault Rifle.
Unfortunately for the "quickscopers", this weapon is a no-no for quickscoping, even with it's high power, it's slow reload and high recoil set it off for a mischeavos use.
The weapon is unlocked at level 42 and can be bought for $2000. It is extremely advised not to buy or use, as it would be worthless to use it in later rounds.