Left 4 Dead 2 primary weapons as secondary weapons. If you use one of these secondary weapon mods, it will disable that weapon from the primary slot replacing your pistols. If you want to use a particular weapon as secondary and a friend wants to use another, you need to disable consistency checks on the server.

REMEMBER TO DECONFLICT MODS AND DISABLE CONSISTENCY CHECKS (i.e. sv_consistency 0) if multiple players use different weapons. I would not recommend using more than one or two at most at a time. Remember, these will move the selected weapon to a secondary slot, removing it from the primary slot.
If you are subscribed to one of my Mk II mods, you need to disable that in your mods as all of these are Mk II variants with the exception of the M-79, which is the Mk IV.

MAKE SURE TO SET sv_cheats 1 IN THE CONSOLE FOR THESE TO WORK!