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I recently had an opportunity to take a look at a PS90. These are the extended-barrel, semi-auto only version of the FN P90. Aside from the lengthened barrel, barrel shroud, and modifications to the fire control group, they are identical to P90s. The P90 was the correct idea at the correct time for FN Herstal. Prior to its introduction, they had been focused on the BRG-15, a 15.5mm very heavy machine gun: This potent weapon fired a monstrous 15mm, later 15.5mm round that edged out the 14.5x114mm round for raw power. The idea, as I understand it, was that Conventional Forces Europe treaty restrictions limited the number of vehicles armed with autocannons that signatory nations could possess. Heavy machine guns were substantially less restricted. While the BRG-15 had a number of desirable features, such as selectable dual ammo feed and APDS ammunition, it did not sell, and FNH needed badly a new, hot product. Enter the P90. FN had been working on a new type of ammunition for a PDW for some time. Their initial ammunition concepts had been quite exotic, featuring APDS projectiles fired from a rimfire cartridge case: FN had grasped that a modern army is mostly support personnel; mechanics, cooks, clerks, etc, and actually only a minority are front-line combat troops. If they could sell a weapon to arm second-line troops, and if they could get this new class of weapon to be NATO standard, they would be fat and happy once again after all the money they'd wasted on the BRG-15. H&K thwarted them on the NATO standardization front, but FN did get comfortably back in the black, as the P90 has sold quite well. The design of the P90 is, shall we say, heavily inspired by the Steyr AUG. The AUG features a cast aluminum receiver, a polymer stock which houses the trigger, fire control components and magazine well, and a bolt carrier group that slides on two guide rods with internal return springs. This is the aluminum receiver of the P90: Here are the telltale porosity marks of casting: Here is the polymer stock, showing the magazine well and some of the components of the trigger group: You can also see that there is a seam between the two stock halves; wide enough to admit sunlight in some places. And here is the back of the stock, showing the hammer pack taken out of it: Finally, here is the bolt, which rides on twin guide rails with external return springs. I guess that's a little different: The P90 is a simple blowback design, which greatly reduces the cost of manufacture because there are no breech locking components that need to be made to tight tolerances or made of high-quality material. However, this does place some constraints on the ammunition design. Here are the sockets in the receiver, astride the barrel breech where those rods fit into: FN wasted hardly any time coming up with something new; they simply put their new 50 round magazine into a straight-blowback AUG, and set about selling it. There is actually a lot about the design that suggests it was somewhat rushed. Late in the development of the P90, the ammunition was shortened by a few milimeters. Instead of completely redesigning the whole gun and magazine: The magazines simply feature an indentation that fills the space the original, longer rounds used to. From a production engineering standpoint, the P90 has many admirable features. It has a minimum of machined features, and the machined parts are very simple shapes. The receiver is cast with some machining to final dimensions, and the stock is injection molded. Casting and injection molding are both well-suited to cheap, high-volume production. Parts that would be machined in other designs, such as the fire control lockwork and hammer, are injection molded in the P90. The P90 also features a number of parts that are semi-permanently affixed to larger assemblies, and can only be serviced at an armorer level: The charging handle and the charging handle return spring are looped around the barrel, and are not user-serviceable. The trigger transfer bar and magazine catch components are sandwiched in-between the two stock halves. The return springs cannot be de-mounted from the bolt either. This is actually typical of non-US designs; the degree to which the end-user can service and modify the weapon is limited to basic field stripping. I remember being shocked at how fiddly and difficult it was to pull the bolt from the bolt carrier group in a TAR-21, only to find out later, reading the manual, that this is supposed to be an armorer level repair. Apparently the bolt cannot even be removed from the bolt carrier on the new Cz 805 without tools: To American shooters, the P90 comes off rather poorly. It doesn't disassemble as far as they would expect, and it looks very slapped together utilitarian. For FN it was exactly the right design at the right time. It had wide appeal and could be made in massive quantities cheaply.