X Products warned their customers that the ATF evaluated the Can Cannon and determined that it would be considered an SBR if placed on a rifle receiver, and an AOW if used on a pistol AR. See the ATF letter on the Can Cannon. The ATF has defined the Can Cannon as a shotgun. However, the device by itself is not a firearm and is perfectly legal to own. Attaching it to a firearm is illegal unless the user has the proper federal licenses.
If you have an SBS, you can add the Can Cannon as an additional configuration to your SBS by using this Additional Configuration Form we have prepared for use with our Gun Trusts, SBSs and multiple configurations. Here is a link to using multiple configurations with a Form 1 for those not familiar with it
X Products claim to be working on a modification which may eliminate this need, but for now we would recommend that any of our clients using a Can Cannot by X products either use a Form 1 to build the appropriate NFA item or add it as an additional configuration to an existing SBR to be used with an SBR that has already been registered. If you do not have a Gun Trust, contact us for more information on how forming a well drafted Gun Trust can permit you to legally use your Can Cannon and protect others from potential violations of the NFA.