AR57 Upper Accepts 50-Round P90 Magazines

The 5.7x28mm upper can be installed on any AR lower, turning the magwell into an ejection port.

A 57 Center AR57 upper
This photo from Reddit shows a 57 Center AR57 upper installed on an AR lower receiver. The custom upper accepts FN P90 50-round magazines. Spent casings are ejected down through the un-used magwell.photo from reddit.com

Ever want to see what happens when you smoosh together an FN P90 submachine gun and an AR-platform rifle?

It’s easy, and you can do it, just by buying a custom upper receiver.

The gun above, posted by u/nobodyishomerightnow on Reddit, is built with a 5.7 Center AR57 upper receiver and, according to him, is "what happens when an AR and a P90 got really drunk, did things, and had a child out of wedlock."

The upper is a complete receiver chambered for 5.7 x 28mm ammo, like the P90. It also accepts the unique, low-profile, 50-round P90 magazines.

The ammunition is fed from the magazine above, situated so that it’s above the barrel for the best weight distribution. The spent cases are ejected straight down through the lower’s un-used magwell, making it great for lefties.

There’s no gas system, instead the 5.7 upper has a blowback operation with a free-sliding bolt, which is the only moving part.

The upper includes almost three feet of Picatinny rail space and vent holes for cooling during sustained fire.

The free-floating 16-inch barrel includes a proprietary multi-port flash suppressor and comes with a “correct-weight recoil buffer” tuned for 5.7 ammo.

The Gen 2 model has a fluted barrel, saving weight, and an updated charging handle, ambidextrous mag release, and an extended rear accessory rail for the mounting of a wider variety of optics.

A look at the AR57 upper on its own.
A look at the AR57 upper on its own.photo from Brownells.com

So what are the advantages of such a franked-gun? Well, for one thing you can have 50 rounds of light recoiling, speedy 5.7 ammo on board without toting around an awkward and huge box magazine or drum. You hardly even notice the P90 magazine is there.

The only production firearms chambered for the 5.7x28mm cartridge are the P90 and the Fn Five-seveN handgun.

The upper receiver retails for about $920.