Bugs: 1. The under barrel attachments do not always resupply correctly. It can take a great deal of weapon swapping emptying clips and reloading to get them to resupply 2. The attachment will not always deactivate, sometimes the weapon will refuse to return to primary barrel until you switch to a third weapon Exploits: 1. Once you know the proper way to cycle weapons you can carry additional Grenades/Shotgun shells, up to the maximum amount of bullets it is possible to chamber in the main barrel. I think all of these have to do with the game confusing the two barrels, it does not see them as two separate weapons, or two parts of the same weapon. It sees them as the same weapon in some instances.
Are you sure that's an exploit? This issue has been with me since I joined beta and I've never figured out whether or not grenades or shotguns were meant to be reloaded. Mix in this issue with grenades not actually causing damage and you have a mess relating to under barrel attachments.
