The FN P90 submachine gun (SMG) was developed in the late 1980s as a personal defense weapon for the troops whose primary activities does not include small arms, such as vehicle and tank crew members, artillery crews etc. Standard pistols and submachine guns chambered for pistol rounds were proved ineffective against enemy soldiers, wearing body armor; Therefore FN designers first developed a new round with enhanced penetration, initially known as SS90. To achieve necessary high penetration while keeping recoil impulse low, FN used a small-bore approach, creating a round that looked much like the scaled down 5,56NATO round. It must be noted that similar concepts were tried in other countries, most notably in USA, several decades before FN; for example, US Army tested M1 carbines chambered for .221 Johnson Spitfire round back in late 1950s; later on, Colt produced its .22 SCAMP and 5,6x30 MARS ammunition for special SCAMP machine pistol and MARS "mini assault rifle" (a scaled-down M16 rifle) respectively. It must be noted that 5,6x30 MARS round was in a sense a direct predecessor to 5,7x28 FN SS90 round, although the latter featured slightly lighter and faster bullet. By late 1980s a concept of a small-bore, low-impulse "personal defense weapon" (PDW) with good accuracy and lethality at ranges of up to 200-250 meters was well established, although there were no weapons adopted for service yet. FN decided to follow this concept and to create its own PDW using clean sheet approach. Basic ideas used for this development, designated as "Project 9.0", included the following: minimal size and weight of weapon; large magazine capacity; complete ambidexterity; ease of use and maintenance.
To save on size and weight, FN designers put new weapon into compact and lightweight stock of bullpup layout, made of impact-resistant polymer. The high-capacity magazine also was made from semi-translucent polymer, and holds 50 rounds in two rows. To made loaded weapon as compact as possible, FN designers followed the idea of American designer Hall, and placed the magazine above the barrels, with cartridges stored in horizontal position with bullets pointing to the left. While Hall system employed a rotary feed unit, operated by the bolt, to put new cartridge in line with the barrel, FN designers incorporated a stationary helical ramp into each magazine, which rotates cartridge for 90 degrees prior to placing it into feed lips. Complete ambidexterity was achieved by using ambidextrous controls (including dual charging handles and dual back-up open sights), and bottom ejection. Finally, simplicity of aiming was achieved by use of integral reflex type collimating sight and integral laser aiming module (LAM).
Resulting weapon appeared in around 1990 as FN P90 personal defense weapon, along with improved 5,7x28 SS190 ammunition, which replaced polymer-cored bullets with heavier dual-core (steel / aluminum) bullets with better penetration against body armor. Several other types of ammunition were developed for this weapon, including tracer, subsonic ball and soft-core training ball.
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