Submachine guns are not generally the most popular kind of firearm amongst civilians, but the FN P90 (also known as the FN Project 1990) is simply too special a gun to not warrant near-universal demand. While it’s often said that the submachine gun is a more compact version of the machine gun, this isn’t necessarily the case – it would be more accurate to say that the submachine gun is actually a larger, more powerful version of the handgun. Submachine guns, or SMG’s, are essentially machine guns which use handgun ammunition rather than rifle ammunition. Is the FN P90 Legal In The United States?.Design and Ammunition: How Many Bullets Does it Hold?.