Different weapons have different damage ratios, dropoffs, ranges, accuracies, etc etc. However, if you mix different weapons together, you get something new. Let's go over that.
The pistol is a jack of all trades. It has a 12/36 SCAR (this is the term I'll be using for Start Clip-Ammo Ratio). It has OK damage output and great range. Its alt-fire is a real precise scope, which has on the spot zoom-in and essentially no tradeoffs. If you shoot something, it's better if you should zoom in directly before you shoot, and immediately zoom out. Your general playstyle with this will be: Long range sniping, short-range avoidance. It's favored for newcomers who need to become familiar with the essentials of the game.
The shotty is a short-range-combat weapon. It has a 6/12 SCAR. It has great damage output, yet not-so-great range. One mechanic that the shotty has is pushback, which takes into account half-square movement on ground and one-square movement on air. This one/half square, however appears to be little on paper, is extraordinary for small gaps that are just out of your reach. Its alt-fire is camouflage, which enables you to mask as a three-tall column for the squares alongside you. With corners, this is somewhat buggy, however it's great once you settle in. Your general playstyle with this will be: Close-range combat, being a sneaky boy. It's favored for people who want a solid damage dealer.
It's... a sniper. 4/8 SCAR. Long range. Scope. Gimmicks: It's a goofing sniper. You know what to do with this.
The machine gun, rather called the SMG for gamers, is another short-range-combat weapon. It has a 40/120 SCAR, It has a 40/120 SCAR, the greatest yet littlest of the bundle. It has not really good damage output or default range, but rather to redress, it has the second-quickest firerate (not including clip dump) and is the only auto-fire weapon. The gimmick that it has is its accuracy diminishes the more you shoot, achieving max at just 4 shots. Alt-fire is just more dreadful. Mouse2 is Clip Dump, which spreads the accuracy beyond the default crosshair. The camera shakes as well, to make it even more terrifying. However, with its amazingly quick firerate in CD (clip dump), it bulldozes all firerates of some other weapon, excluding turbofire. Your general playstyle with this will be: Getting up close and personal, hit-or-miss shots (no pun intended). It's favored for people just want a gun.
Pistol / Shotty
Good damage output, featuring long range, short range, and in the middle.
Pistol / Sniper
Very good at long range, decreasing good-ness as you get closer.
Pistol / SMG
Amazing at close range. For long range, if you run out of bullets from your pistol, use the SMG and fire in short-controlled bursts.
Shotty / Sniper
You're gonna need real luck to hit perfect headshots every time. Other than that, good long-and-short range capabilities.
Shotty / SMG
The build for close-combat. Switch to your SMG after firing a shot from your shotty, then switch to the shotty after a few shots from the SMG.
Sniper / SMG
Good at aiming? Want to test your luck and skill? Use this.