Rewolf put a lot of work into the weaponry. Here the alt-fire key doesn't fire the gun a second way. It lets you access special features. A shotgun can be modified this way to expend 1-4 rounds per shot and fire it using shot, riot pellet or rifle. The laser pistol can be used in rapid-fire mode, single shot or a shot that explodes.
Grenades can explode on impact, proximity, or with a trigger.
All the weapons have several uses and learning to use them all is worth the effort when the action heats up. Sierra promised us vehicles in the game, which would have really made it stand out because that's never been really successfully done before. But instead we have a Tank. You'll occasionally run out of gas and have to leave the tank to find more, but you can't help but think this cool idea could have been implemented much better. All of the above is just reminiscent enough of Half-Life to put a smile on your face but different enough to keep you playing, which leads directly to Gunman's flaws.