The FN-P90 was designed and is created by a company called Fabrique Nationale, who also designed the ammunition for this weapon, the FN 5.7mm round. The ammunition's purpose is to provide armour piercing capabilities.
The afforementinoed 'unconventional nature' of this weapon is the result of the manufacturers effort to reduce the physical profile of the weapon. Compared to conventional weapons, the firing mechanism and chamber can be considered as part of the butt and the magazine rests on top of the weapon, lengthways along the barrel. The weapon itself can be considerd to be of a polymer construction (there's more plastic in this weapon than metal), and also boasts a set of ambidexturous iron sights alligned to the left and right of the scope with classic red-dot reticule.
The overall appearance of the weapon shares similarities with the Steyr AUG, with regards to an ergonomically designed set of hand grips and stock. Similarities don't stop there however, the P90 uses the same pressure sensetive block-trigger which fires a single round when depressed half-way, and full auto when fully depressed. The selector is mounted below the trigger and offers 'safe', 'semi-automatic' and 'automatic' settings.
The most extraordinary thing about this weapon is the way the magazine is mounted. In many typical automatic weapons the rounds in the magazine are aranged so that the bullets are pointing in the direction they would be in as they are resting in the chamber. This is for ease of chambering the next round. The FN-P90 differs from conventional weapons as the bullets lie in a direction perpendicular to the barrel itself. Here the magazine is placed so it lies on top of the barrel wheras typically submachine guns have a seperate portrusion for the magazine. The weapon uses a simple blowback mechanism.
This means the loading mechanism actually spins each round through 90 degrees before it is positioned in the firing chamber. While still managing to keep the physical profile of the weapon small, the P90 manages to cram a whole 50 rounds of 5.7mm into the magazine, 20 more rounds than your typical MP5.