The concept of the XR-9 pistol is to provide the ballistics (penetration and terminal effects) of the full-size pistol in sub-compact pistol, suitable for concealed carry. The XR-9 was initially designed in 2004 and is in its third prototype as of now. Boberg Engineering developed a method of lengthening the barrel by changing the feed mechanism. Unlike most semi auto pistols, the XR-9's design extends the barrel further back into the slide and places its breech area and chamber above the magazine, instead of being located in front of the magazine.
Look at the photo below and note the feed claws that grip the topmost cartridge in the magazine. In autoloading, the claws pull it back and up for loading into the breach. Boberg used a "push- pull" system similar to the one that was used on the Gabbet-Fairfax Mars pistols of the early 20th Century. A related system is used in some older type belt fed machine guns such as the Maxim, Browning and PK. Boberg claims the XR-9 can provide an increase of muzzle energy of about 25% compared to pocket pistols of the same size and caliber.
Look at the feed claws gripping the cartridge before pulling it back and up for loading into the breech
The Boberg XR-9's trigger is double action only (DAO) and it is hammer-fired. The action is short recoil operated. The two-stage "pull-push" feed system centers around the claw-shaped loader, which to the slide on a pivot. When the slide is cycled the the claw pulls the cartridge rearwards from the magazine and at the end of recoil stroke, claws are lifted to place the cartridge to feed position. On the closing stroke of the slide, cartridge is pushed into the barrel chamber, and the feed claws are lowered to grip on the following round in the magazine.
The XR-9's barrel is about 1.2inches longer than the Kahr PM9, with an approximately even overall length