* I assume you're already familiar with the basics of creating your weapon, from the graphics to importing them into your WAD file, to coding it up in Decorate and aligning everything.
* I'm going to be mentioning a lot of abstract concepts (many of which I may or may not have just made up on the spot). Bear with me. You might understand what I'm talking about.
Alright. So you've got yourself a kick-ass new shotgun for your mod, it's all graphic'd up and you've got a sweet firing sound and everything. But you get in game and find that you just don't have much fun using the weapon as you did with some other guy's shotgun from another mod. A few people might think that the solution is to just toss the shotgun they made and rip the other guy's instead, but this tutorial's purpose is to show that you don't need to give up. Here's a few steps that I tend to take if my guns aren't fun to use.
Step 1: Check the concept.
In a lot of cases this won't be a problem, but if you're creating a weapon that hasn't been made before (like a swarming plasma gun or something), maybe the very idea of the weapon needs work - perhaps your gun sounded cooler on paper than it does in practice, maybe some special weapon effect you planned just doesn't work very well in the engine (like a resurrect gun or magic ricochet bullets). But like I said before, in a lot of cases this won't be a problem, especially if all you're making is a shotgun.
Step 2: Weapon Power to Sound Power ratio.
A little concept I figured out while playing STALKER: one of the things that makes a gun fun to use is the weapon's power compared to the perceived power of the sound it uses when it fires. Say you've got yourself a sweet new magnum, capable of killing an Imp with a single shot, a Demon in two shots, and so on. The sound you've chosen...well, it sounds like a pack of Fun Snaps or some guy hitting a wall with a hammer. That's just not going to work. If you want your weapon to feel appropriately powerful, you want your gunfire to have the right volume (gunfire tends to be LOUD, louder than most other parts of the game), appropriate reverb (Doom's primarily an indoor game so let's not go overboard on the outdoorsy echo), and of course the right amount of punch (hint: gunshots don't tend to "fade in"). It's a tricky concept to work with, but as long as the "power" of your weapon sound matches closely enough with the power of the actual gun, you'll be fine. (Let's also make sure this doesn't go the other way around: a 9mm peashooter with the sound of a nuclear bomb? Uh...)
Step 3: Add some "weight" to your weapon animations.
People aren't computers. When we move our limbs about, we tend to do so with a small degree of inertia; the way the universe works will tend to make it difficult for us to move at a constant speed with no inertia whatsoever. So when I animate a weapon's reload sequence, I tend to work with a sort of inertia: longer, drawn-out movements will start out slower near the beginning of the movement (say a hand coming up to the shotgun to load a shell into it), speed up a little in the middle, then slow down near the end (as the hand will need to slow down in order to not miss the insertion port). Here, have a (cheesy) graph:
The numbers indicate the position of the hand on screen for that frame (assuming that the hand will take 8 frame to reach the shotgun, and of course assuming that the shotgun is on the right side of the screen...no discrimination against you lefties). If every frame of that sequence has the exact same duration in tics, the perceived speed of the hand's movement will still appear to change, and appear less robotic and artificial because it's not moving at a constant speed. Get the picture? (I hope so, because I can't figure out how to explain it further.)
This same technique may also come in handy for other animation sequences, like on-screen weapon recoil.
Step 4: Quality of resources.
Of course all of the above might be a moot point if your sprite looks terrible. Let's face it: not everyone is an artist, and I don't expect everyone to be, but when your mod is comprised mainly of Doom-style weapons and the weapon you've just added looks like an MS Paint sketch, that's going to be pretty distracting and make your weapon less fun to use (see my own Doomam.wad and its terrible mass driver weapon). Sometimes, you just need to toss your resources and get/make new ones.
I hope these little guidelines will help someone.