Bushmaster has changed the design and parts used to build the new Carbon 15 rifles and pistols.
They now have 5.56mm chambers rather than the tighter chambers used before which helps with the extraction problems that were experienced before. The gas ports have been changed as they were oversized causing the systems to run too hard leading to cycling problems and had too much felt recoil. Buffer recoil pad has been changed so the pads do not come off under fire causing cycling problems. The old barrel extensions would not allow the use of an AR15 bolt. They now use standard AR15 bolts and barrel extensions as they were producing bolts and extractors out of the wrong material which was prone to breakage. The lower receiver parts are now Bushmaster AR15 parts which are of a better quality than the aftermarket parts that were being used.
The upper and lower receivers will now interchange with AR15 receivers so there are more options available to shooters. It does require that the same bolt carrier and buffer system used with the lower receivers be used if interchanging upper or lower receivers. For example if you place an AR15 barreled upper on a Carbon 15 lower you must use the shorter bolt carrier and buffer system. If you place a Carbon 15 barreled upper on an AR15 lower you must use the AR15 bolt carrier.
Due to Federal Firearms Laws rifle uppers can not be installed on pistol lower receivers and pistol uppers can not be installed on rifle lower receivers.
Other upgrades and options are on the drawing board and will be announced when they are available.