I understand your frustration and do consider this exploiting. Unfortunately there is no way to block the use of the M26 as of right now. This is due to the fact that the game server does not report back kills from the M26 when under slung. This means that if you get a kill with the M26 while on say, an M16A3, the server reports back the kill as a M16A3 kill. We have admins on throught the day, as well as a reporting feature in game. You can also file complains and screenshots here in our forums. Other than that, there is no way for us to counter this automatically short of banning all weapons that can use under slung attachments.