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The AR57 is a replacement upper receiver that allows a standard M4 to be converted to 5.7x28mm and use a FNH USA-style 50-round mag. The empty cases are ejected through the mag well.

Initially researching AR57’s M4 conversion, I had to go to their website to see what everyone was talking about—because somehow this product has been flying under my radar for some time. I was aware of FNH’s 5.7x28mm cartridge, the modernistic P90 PDW and the Five-seveN pistol. After perusing AR57’s website, the concept of a 5.7x28mm upper on an M4 really intrigued me. I contacted the company and spoke to Markus Laukers, who began educating me on what I had been missing.
First came the cartridge. FNH USA developed the 5.7x28mm as a new CQB cartridge that could effectively penetrate body armor without over-penetrating soft and hard targets. The cartridge was introduced in the P90 platform, followed by the Five-seveN pistol. The first loading of the 5.7x28mm was known as the SS190. When fired from the P90 PDW, the SS190’s 31-grain projectile had a reported velocity of around 2,300 feet per second (fps). Due to the armor penetrating capabilities of this round, distribution was limited to military and law enforcement agencies.

ar57s-m4-conversion-e28094-501-firepower-bThe 50-round mag must be inserted into the front rail and then seated above the chamber, limiting the space for optics and other accessories.

The AR57 conversion is a complete upper that will fit on any standard M16/M4 lower receiver. The one-piece upper is machined from aircraft-grade aluminum and features integral Picatinny rails at the 9, 3, and 6 o-clock positions. Two raised rails, one forward and one at the rear of the receiver, allow the use of fixed sights or optics —depending on the user’s needs. The action is a straight blowback with no locking lugs on the bolt. A non-reciprocating charging handle is located on the right side of the receiver to chamber a round or clear the weapon. Due to the pressure and bolt travel of the AR57, a shorter recoil spring buffer is needed to ensure reliability. The non-tapered barrel is 16.04 inches in length and is equipped with a flash hider that is threaded in a 1/2×28 pitch. This allows the AR57 to accept most screw-on suppressors and muzzle brakes.

At the heart of the AR57 is the FNH-style magazine. FNH USA shook the firearms industry when they introduced a feeding system where the magazine is mounted parallel to the barrel and rounds sit at a 90-degree angle to the barrel. The base of the magazine consists of a round feeding platform and contains a spiral feeding system that aligns the round with the chamber. While it sounds complicated, it is actually very simple and has proven reliable.

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The AR57 is a replacement upper receiver that allows a standard M4 to be converted…