You mean what does it stand for? It stands for "Adaptive Combat Rifle".
You have to get all of the attachments on the ACR to get the mastery.
Level 48
The assualt rifle Acr is unlocked at level 48.
The ACR or the AK-47 they are excellent
ACR and Desert Eagle Sleight of Hand
Modern Warfare 2
Red dot, grenade Launcher, FMJ, or an ACOG sight.
scar is better because it has better accuracy and damage and rate
the easiest weapons to master are the scar-h, acr, tar-21,famas, vector, m4a1,and the ak47
well.... i use the famas, tar21 acr, and ak47. the famas is a 3 round burst but it kills your enemy in two or three shots.
Modern warfare reflex and modern warfare are the same. Modern warfare "reflex" means that its for wii. If its just modern warfare its for the other consoles
Aim down sight.