Call of Duty: Modern Warfare players have reported that the new Rytec AMR sniper rifle, added in Modern Warfare Season 4, isn't leveling up as it should. Unable to level up the weapon, those players can't unlock attachments and skins for the gun.
Infinity Ward developer and Call of Duty co-design director of multiplayer Joe Cecot explained the glitch appears to be tied to explosive rounds on the Rytec AMR. To avoid losing out on weapon experience, players should un-equip explosive rounds until Infinity Ward can institute a more permanent and less disruptive fix from its end.
The Rytec AMR has proven particularly bug prone for reasons that are not immediately clear. A more complete fix, plus fixes for the gun's many other bugs, will likely arrive in the next Modern Warfare update, scheduled to arrive in-game in the coming days.