Monolith Arms P-12

The Monolith Arms P-12 is an American prototype shotgun developed by Brian F. Abbott in 2005 under the banner of Monolith Arms.^[1] It was designed as a bullpup, pump-action shotgun with a unique 20-round detachable box magazine, inspired by the design of the FN P90. The P-12 never reached mass production and remains a singular prototype in private possession.^[2]

The P-12 was developed to offer a high-capacity, compact shotgun suitable for tactical and defense use. It featured a bullpup configuration, which allowed the overall length of the firearm to be shortened without reducing the barrel length.^[3] The primary innovation of the P-12 was its top-mounted, horizontally positioned magazine, similar to the FN P90, allowing for a high-capacity feed of 20 rounds of 12-gauge shells.^[2]

Key specifications of the P-12 include:

• Type: Shotgun
• Caliber: 12-gauge
• Action: Pump-action
• Overall Length: 26.3 inches (67 cm)
• Barrel Length: 18 inches (46 cm)
• Weight: 8.0 pounds (3.6 kg)
• Feed System: 20-round detachable box magazine

The P-12 incorporated a blend of custom-made and off-the-shelf parts, including standard fire control group components and Saiga shotgun magazine springs. This approach was intended to facilitate easier maintenance and compatibility with existing firearm components.^[1]

The P-12 was primarily constructed using 3D printing technologies, making it one of the first firearms to utilize this manufacturing method for prototyping.^[1] Fused filament fabrication (FFF) and Fused deposition modeling (FDM) processes were used for components like the receiver and magazine housing, while metal components such as the bolt and barrel were likely produced using traditional machining or direct metal laser sintering (DMLS).^[3]

The P-12's top-mounted magazine posed significant design challenges, particularly in ensuring reliable feeding of rimmed 12-gauge shells in a double-stack configuration.^[1] The magazine system was eventually made functional, but the overall design still faced challenges with brand alignment and market acceptance.^[2] Despite interest from a major firearm manufacturer, the P-12 was ultimately shelved and never entered production.^[3]

Although the Monolith Arms P-12 was never commercially released, it influenced the development of high-capacity, bullpup shotguns that appeared later on the market, such as the Kel-Tec KSG and the UTAS UTS-15.^[2] Its early use of 3D printing for prototyping set a precedent for future firearms development and rapid prototyping in the industry.^[1]

• Kel-Tec KSG
• UTAS UTS-15
• Standard Manufacturing DP-12
• FN P90
• List of 3D printed weapons and parts

• The Firearm Blog: Monolith Arms P-12 Overview
• AR15.com Forum Discussion on Monolith Arms P-12
• Instagram Post by Brian F. Abbott