Pull trigger once: Machine Gun
Pull trigger twice: Shotgun
Pull trigger three times: Grenade
Note: There are also limits to the Special Weapon ammo, and additional special ammo is acquired in specific increments for each weapon, as below:
Machine Gun: Ammo Pack; 15 per increment, limit 200
Shotgun: Ammo Pack; 5 per increment, limit 50
Grenade Launcher: Ammo Pack; 1 per increment, limit 5
Note: The Grenade Launcher is the slowest firing gun (and as such requires precise aim for moving targets), followed by the Shotgun (due to its pump time). All special weapons do not have to be reloaded.