This is an weapon mod by an unknown creator, It's a file that adds in several versions of the bullpup FN P90 Machine gun with silencers, lasers, or with scope zoom lenses. This file isn't by me nor do I take credit for it, This file came off the Swat 3 reunited website. Reuploaded here to get interest in the game again.
Put the file in Sierra/Swat3/mods. Enable via the swat 3 mod launcher, and the file will be ready to use for single and multiplayer.