Jan used these two guns during the Valentine Brother's raid on the the Hellsing compound. He takes down dozens of soldiers using these guns, easily slaughtering the ones his Ghouls hadn't already killed. They are later abandoned during the fight between himself, Walter C. Dornez, and Seras Victoria.
The real-life P90, a personal defense fire-weapon (PDW) manufactured by FN Herstall in Belgium, was developed somewhere between 1986 and 1991. It features several innovations, such as a top-mounted 50-round transparent 5.7x28mm magazine and ambidextrous controls, notably taken advantage of by Jan when he wielded one of them in each hand. It is also notable that the ejection port for shells is located on the underside of the gun. Jan's modified version seems to have a suppressor as well, but neither the sound of the gunfire nor the amount of damage seems to be suppressed, as it sounds similar to a standard assault rifle when fired and tears through the Hellsing soldiers. However, it must be considered that Jan emptied a considerable amount of bullets into them.