This "Gun" is used only during the final boss fight of Resident Evil Outbreak. It can be found during the battle on the exterior hull of a non-functional Tank.
In Resident Evil Outbreak File 2, George Hamilton begins each scenario with this weapon in his inventory. It can be used in conjunction with one of his special abilities, firing the various pills he can make from herbs. If mixed with an anti-virus pill it can kill a zombie in one shot.