Well, it kinda depends on the nature of the FPS. Some (like UT/Quake 3) have weapons like plasma cannons and stuff which would look best as a plain that allways points at the camera. Others, like Ghost recon/Half Life use more realistic projectiles which can barely be seen because of the speed and size. For these cases, a long ghosted plain with a texture that makes it look more like a streak would look fine. I'd prefer not making any bullets at all. Only a barrel flash and sparks (for when the bullet hits something).
Morfy's Law - Enythink thet ken go rong willll.
Cole's Law - Thinly sliced cabbage.