How To Turn Your AR-15 Into A P90

P90 overview and 5.7x28 cartridge

The video opens by comparing the compact FN P90 and PS90 to an AR-15 configured to mimic their role. The P90 is described as a long-standing and popular personal defense weapon dating back to the 1990s. Despite its unconventional appearance, its ergonomics are noted as comfortable, with controls and sighting systems lining up naturally. The host highlights that the platform is chambered in 5.7x28, a fast-moving, small-caliber cartridge. A P90 magazine is shown with 5.7x28 rounds visible inside, emphasizing the unique top-mounted magazine design. This sets the stage for discussing how shooters who already have 5.7x28 ammunition and like AR-15 controls might benefit from an upper that blends these characteristics.

Introducing the AR57 ULT upper receiver

Attention shifts to the AR57 upper receiver with an integrated barrel, referred to as the ULT, or ultra light tactical upper. It is built from 7075-T6 aluminum and is presented as a newer generation of a concept that has been on the market for several years, likely a second-generation model. The design allows use of standard AR-15 components such as the stock, pistol grip, safety, fire control group, and even the lower receiver’s magazine well. However, AR-15 magazines do not lock into this upper and cannot chamber because of the different caliber and system, preventing accidental misuse. The AR57 upper is intended to run P90-style magazines on top while retaining familiar AR-15 handling and layout for users already invested in that platform.

Controls, ejection, and ergonomics

The AR57 upper is described as a direct blowback design. It features a right-side charging handle that folds forward when not in use, reducing protrusions along the side of the rifle. The host notes that, similar to the P90, spent cases eject downward through the AR-15 magazine well area. That opening effectively becomes the extraction and ejection port, mirroring the P90’s bottom ejection concept. This approach allows the rifle to maintain AR-15 ergonomics while adopting the P90’s magazine and ejection style. The familiar AR-15 stock, grip, and safety controls remain in place, making the setup intuitive for shooters accustomed to standard AR-15 rifles but interested in the 5.7x28 cartridge and P90 magazine system.

Magazine capacity and accessory options

The discussion turns to shooting characteristics of 5.7x28 firearms, including the FN Five-seveN pistol and Ruger-57 pistol, which are described as enjoyable to shoot with notable magazine capacity. The P90’s 50-round magazines are highlighted as a major advantage, even though they can be somewhat tedious to load. Their loading method, where rounds are pressed in and rotated, is noted as mechanically interesting, and the transparent bodies allow easy viewing of remaining ammunition and how rounds are stacked. The AR57 upper leverages these 50-round P90-style magazines to deliver high capacity without the bulk and weight of 60- or 100-round AR-15 drum magazines. The upper includes M-LOK rail sections and a Picatinny rail up front, providing ample mounting options for accessories such as lights, grips, and optics within the familiar AR-15 ecosystem.

Live fire, reloads, and reliability

On the range, the host loads a reduced-capacity PS90 or P90 magazine, described as a neutered version holding around 25 to 30 rounds. The magazine is inserted on top, pressed down into place, and the rifle is charged using the forward folding charging handle. The host cautions not to confuse the magazine release with the charging handle, as they are in different positions. After firing, the magazine is removed by pushing the release, allowing the mag to pop free for retention and a quick reload with another top-mounted magazine. The rifle is shown running in a binary trigger mode with a few remaining rounds, and the cycling is described as very reliable. The shooting sequence reinforces that the AR57 upper handles the 5.7x28 cartridge smoothly while maintaining controllability and high capacity.

Disassembly and ambidextrous features

After shooting, the ground beneath the shooter is covered with 5.7x28 brass, illustrating the bottom ejection pattern through the AR-15 magazine well. The host demonstrates disassembly by separating the upper and lower receivers and tilting the AR57 upper so the bolt carrier group simply falls out. The internal mechanism is described as very simple, with an arrow marking on the bolt carrier group indicating the correct orientation for reassembly. The rifle is then reassembled by reattaching the lower receiver and pushing the takedown pin back into place. The magazine release on the AR57 upper is shown to be ambidextrous, allowing operation from either side. This configuration supports both left- and right-handed shooters when it comes to magazine changes and basic handling.

Left-hand charging setup and final thoughts

The host explains that, in the current configuration, the charging handle is on the right side, favoring right-handed shooters. However, the AR57 upper allows the charging handle to be moved to the left side. By removing the bolt release and loosening the Allen screws that secure the charging handle assembly, the user can reposition the handle for left-hand operation. This requires some additional work but offers flexibility for left-handed shooters who prefer a side-charging setup on their dominant side. The host notes that reliability with the AR57 upper has been impressive. For shooters who already own multiple AR-15s and possibly a P90, the AR57 upper provides a way to run 5.7x28 P90 magazines on an AR-15 lower, combining the cartridge and magazine system of the P90 with the ergonomics and accessory support of the AR platform.