Rifle Mod is an Alien Swarm server-only mod that forces players to play with assault rifles only, harder aliens, more ammo, smarter bots. However rifle only restriction can be disabled and a number of modifications can be turned on during briefing, see commands. Can be installed on a dedicated server or on a regular game. See details at "Official Page"
Extract it into Alien Swarm\swarm\bin. Use console commands to activate game modes in each round. rm_mod 0 - classic Alien Swarm, rm_mod 5 - Solo Player mode, you can choose any character and quickly hack doors.