Twilight Sparkle ACR

Use 7zip or Winrar or whatever to navigate to C:\Program Files (x86)\Steam\steamapps\common\call of duty modern warfare 3\main\iw_06.iwd\images\

NOTE: MAKE A BACKUP OF THE ORIGINAL FILE File is weapon_remmington_acr.col.iwi or sumthin'

Drop file of the same name, contained in the DL, in the "images" folder, overwrite if asked, but really, back up the original first for if it fails or something.

start game ... possibly profit!