I remember my police shotgun training had a whole section on how to use specialty ammo. As a rule, you had a tube full of buckshot, for its generally desirable performance, but every so often you had to fire a slug or a novelty round, such as a less-lethal baton shot or a cannister of OC spray.

There is a little series of actions you have to perform to get the specialty shot in there. You load your chosen round into the magazine (The tubular magazine works like a first-in-first-out stack), cycle the action manually to get your "normal" round out of the chamber and your novelty in, then put the normal cartridge back into the top of the magazine. It's pretty cool, and makes you feel like a badass when you do it.

My old Daisy Powerline 880 had a reservoir of BBs, but pellets had to be loaded manually into the action. Something like that could work for your railgun. The sabots of matter that serve as projectiles coul be loaded automatically for rapid fire, but then you could manually open the action, pop the normal round out and replace it with a signal flare, an armor-piercing warhead, or whatever other unusual munition you wanted, then pop that shot off. The next shot, if left to cycle normally, would be a conventianal round.

It's a good way to do something different without swapping out your magazine.

well splinter cell did this with its sc 20k you had sticky shockers bullets airfoil rings excerta,and personally im against this for anything but a stealth game...

for me i like killing someone and gaining there weapon not having 1 weapon and using it for everthing

Are you doing a single player (where the game can be paused) or is it multiplayer?

If you're able to pause the game in any way or maybe even if it is multiplayer I'd do it in between deaths...

Make it where the player can just pop open a menu, just like in Final Fantasy, and based upon what arrays of munitions and ammunitions they have they can create their own weapons out of it.

In fact that could be really cool! You could make it where each type of ammunition has a certain effect (like reagents should in spells), then each gun has a certain projectile method it uses. Bullets should obviously not be used in conjunction with say a rocket launcher without some serious creativity going into it.

But it might be cool just to have the weapon be able to evolve, or let the player actually "enhance" it to do some unexpect things by letting them choose what combination of munitions it can fire.

Then you can either just define what each bullet type does (does it explode? deal direct damage? deal damage over time [biological]? does it deal any damage or is it a flare?) then decide what the guns projectile range is, speed, etc...
Then let the game figure out what should happen, you might see some pretty interesting things happen when you modify a pistol that can fire rockets, lol!

Or take a whiteboard and start drawing a web of ideas, come out with every type of ammunition (you made a comparison to spells so I'm going to say they're reagents) and draw out what they can do. Then make a bunch of bubbles with each gun and what it can do.

Then start drawing lines to connect what each gun can when equipped with what ammunition.

I guess in a way the gun is nothing more then an incanation or the actual invocation of a spell.

The reagents would all determine it's effect... but you'll have to be sure to compensate balance in there.

Then start drawing lines to connect what each gun can when equipped with what ammunition.

You may have to get a rather large whiteboard to do this!

But you've got a really good idea up your sleeve I think.

oh wow... this is eerie. I guess I can't keep my ideas secret after this. In my game design docs, and in a small demo I made 3 years ago I used different ammos with an extremely advanced weapon system. One of the ideas I've always put into my games is multiple ammo types. However you can queue up clips and switch between them. You can also customize each clip loading different bullets that are the same type into a clip. My game is sci-fi too. If you are using shields in your game, make EMP bullets that hurt shields but don't hit for much real damage. Let a player load up a magazine with emp bullets shoot down the enemies shield and switch to real ammo. Also something you should work into your weapons is multiple firing modes. Either using different ammo or the same ammo. When you switch to a firing mode reload that clip. Also have "auto-reload" firing modes, ones that don't stop other firing modes from working.

As a quick example, I made an assault rifle in my demo that had a single fire super accurate shot, and an inaccurate second firing mode that burst shot. It's third firing mode used a different kind of ammo, a rocket which could be loaded while shooting automatically at the cost of lowering the accuracy of the gun while reloading. (I was figuring the character was loading in a new rocket while shooting.)

A good weapon idea I never used would be to have two firing modes with different clips. It was possible with how my clip queue was set up. Right clicking changed the firing mode and the clip queue on the screen changed to show the clips that the player put into that firing mode queue. For instance, in my other example emp bullets on one mode and armor piercing on the other would be cool.

I kind of got lost in micromanagement with my weapon system though. :/ My new one is gonna be better :)

If you have a secondary fire, you could always use that. A gun that fires buckshot in primary will fire buckshot in secondary.

Alternatively, you could have different weapons per ammo type, each with the same general graphic. Minor changes could exist, like the weapon cannister glowing blue or having the words "Incendary Fire Mode" glow from a faceplate. If another ammunition type is in the gun, then you'd just reload. It could work.

Siserian (sp?) in a way its just like switching out canons in the Metroid franchises though. You can add a "wave" spread to your ice beams, or change it to plastma, etc...

I think you can add in some much more random effects and make it an entire focus of a game though and end up with something very interesting. Or make it just one element in a rather diverse game.

It would be very interesting to see just the random element put into it... where you can use any ammunition type and make them "crossbreed" with the weapon. So firing mini-gun clips out of a rocket launcher produces some completely weird effect :)