I know you already "dismissed" the flaregun idea, but just because you think of it as a fire grenade launcher, a pretty retarded idea considered lvl3+ firebug's grenades already set mobs on fire. Think about a hot projectile that partially penetrates a ZED, burning them from the inside as well as setting them on fire. I know a flare probably wouldn't have enough penetration to get stuck inside a body, but it isn't nearly as far fetched as making the xbow bolts go through INFINITE mobs without stopping, loosing power, or even drawing an arc.
Full thread: http://forums.tripwireinteractive.com/showthread.php?p=589897#post589897
Full thread: http://forums.tripwireinteractive.com/showthread.php?p=589897#post589897