I've got this concept for a shooter that I'd like to test out that involves the player taking the most basic parts of a modern firearm(receiver/action, stock/grip, barrel, etc.) and resizing them before combining them all to form a unique gun. My plan to plot all the data out is a spreadsheet with each component of the gun separated and the size changes(length and width) measured on the X and Y axis of a graph. I have sufficient knowledge of how firearms work, but my problem is not knowing the performance changes that come with changing every part(I know most but not all, that is). I also thought about using a point system, where variables like damage, range, and fire rate have their values raised and lowed when changing aspects of the unique gun. Maybe the player should get a number of points to spend on changing part sizes?

 I humbly request input from forum frequenters who have actually fired, studied, or read up on firearms quite a bit. If your knowledge of weapons comes directly from television of video games, please don't try to help with the concept and just comment on whether or not this system sounds fun.

The parts I've been able to list so far are:

Receiver - both upper and lower. The sear and trigger assembly aren't customizable since they probably won't change the variables significantly.
Action - bolt, lever, blowback, etc. The size of this is determined by the receiver's size, but changing the type of action changes the fire rate value.
Barrel - heating of the barrel by firing excessive rounds is not taken into account. Length determines range and bullet deviation from it's flight path(damage, too?) while width determines caliber(damage)(deviation, range, too?)
Stock and Grips - optional, purely aesthetics

Halp.

I'm a kind of gun maniac/junkie so even though I have never fired a gun I have studied them and read up on about how they work. Now when you said customizable guns I thought you meant attachments like laser sight, scope and bigger magazine but it seems you want to customize the inside of the gun. I'll try to help as best as I can as I have also pondered about this idea for my zombie game.

- The reciever can't really be changed unless your talking about assault rifles which can have their upper reciever changed to a RIS rail. So changing the reciever can only apply to some assault rifles.

- When it comes to the rod and recoil spring inside the gun, I've heard that a heavier recoil spring and rod can lessen the recoil but would make the gun a bit heavier although this would only benefit the most for guns that use big calibres such as the Desert Eagle and most likely any .45 ACP handgun. Although I don't know if this would benefit the assault rifles and other guns. This would actually work on all handguns but benefit more for handguns that use bigger calibres.

- The barrel of the gun does indefinitely determine the range, the longer the barrel the more likely it is to hit the target and more likely the bullet will go further, I don't see how it would cause more damage though.

- Stock and Grips do have benefits, Example: a stockless shotgun can have a shotgun stock attached which would make it more accurate. Grips on handguns can also recieve benefits as there are Laser Grips much like Laser Sights except they are on the grip and are more compact which means you can have a laser grip and be able to put it in your holster as well, this is usually a problem with having a big bulky laser sight on a handgun as it will not fit into the holster.

- It is also possible to have a Picatinny Rail on the upper or lower reciever, Example: you can have a Picatinny Rail on the HK416 so that it can have a scope attached. Example: You can also have a Picatinny Rail barrel for handguns which would allow laser sight and flashlight attachments. You would need to change the barrel of the handgun and this would let the player decide whether to have a longer barrel for the range or a Picatinny Rail barrel for the ability to accessorize. Rails can also be put for SMGs, Machine Guns and other guns.

- Sights, certain sights can give certain benefits, Example: Tritium Night Sights allow the sights to be seen in the dark as they glow green. There are also the Match Sights that help increase accuracy, they are mostly used in sharp shooting competitions.

- There are many other attachments such as the Bipod, Foregrip, Pistol Grip, LAM (Laser Aiming Module) and etc. Don't limit yourself to just the inside parts of the gun.

SupremeWarrior: Really great input, you must be worth your weight in firearm knowledge or something. Accessories like rail-mounted sights, bipods, and whatnot would probably only be useful if I went on and somehow turned this into a 3d game, unless I went for a Soldatesque style of gameplay. I'm limited to making a crude 2d platformer to test how the weapon performance would change as I played with the variables. I'm not sure how I would apply accessories to the whole point system, aside from "+1 accuracy" or something like that. Also, this system to primarily be for guns with really simple exteriors, like the Thompson or FN-FAL; mostly guns that require two hands, I guess.Thanks!

Pipcaptor Hatsuya: I really want to focus on the gun aspect of the "game", but striking an enemy with an accessory like a bayonet or buttstock sounds really easy to implement(having no effect on firearm performance, of course). A thank you to you, too.

Ah, I also forgot about Triggers, some people like to customize the gun's triggers, Example: A Colt 1911 can be customized to have a skeleton trigger which is lighter and allows for easier firing, maybe having this on the gun would increase the fire rate because it makes it easier to just spam the trigger.

EDIT: I don't know how deep you wanna go with this firearm thing but you can have some gun maintenance in the game. Some guns are easier to maintain while others would need some cleaning, polishing and etc. If you maintain your gun well enough it will not jam and whatever. By the way what are you going to use to make this game?

Sounds like it'd be too minute of a change in trigger-pulling ease to make itself evident in the game.

There are some guns which can be customized with a kit to make them shoot bigger calibres, this has also been done in Dino Crisis, where the glock used 9mm ammo and once combined with a kit it could shoot .40 S&W which is a bigger calibre and it did more damage that way. Although I don't think there will be many guns in which you can do this.

Bigger magazine means more bullets, that way the gun can hold more in it's magazine.

EDIT: If you could post a list of firearms your planning to use I could give you tips on how you could customize them better.

Well, all the guns would be fictional as they are "made" by the player(if I even get that far in development), but I did plan on letting the style of individual components be changed, like using an MP40's receiver for a gun that uses mostly Thompson-style parts and other combinations like that. As for the magazine, it's size should be automatically changed to be appropriate for the diameter of the barrel, although being able to choose the length of it sounds reasonable to change capacity.

 EDIT: Bullets go in cartridges and cartridges go in magazines.

You can also have different ammo types, if you want! Although I don't know how it's going to fit in with your self made firearms.

Here's a link of some self made firearms maybe it will stir some ideas.
http://www.zombiesurvivalwiki.com/page/Self+Made+Firearms/revision/2

The guns I was thinking of wouldn't be realistically built in any way, but more like firearms that can have .22-ish barrels with Bren-sized receivers attached to them.

EDIT: Most real homemade guns are unique, but a little too unique to fit into a system where parts are essentially identical if not for a difference in size.

Uh, no, I still want the player to have control over the magazine capacity.

Well, we know you are allowed to take certain liberties with how technically accurate these custom firearms are since this will ultimately be for a game. With that in mind, it looks like the goal is finding a simple system in which to alter the attributes of the gun based using those of real-world firearms as a base.

I've actually thought about this problem before while daydreaming of a racing game with a fully customizable auto building system that would (theoretically) be accessible to any level of racing fanatic. (Kinda the same goal as Spore, only marginally more likely to be successful since the systems we are dealing with are much simpler to break down into components.)

The way I would handle it would first build a spreadsheet starting with putting on each row every type of attribute a gun has when firing, that would be relevant to the player as he is using the firearm in the game. (basically anything the user would notice, and therefore care about while building his custom gun) Recoil, ammo capacity, range/accuracy, etc... Then next to each row, using the knowledge of firearms you currently posses, list out every component of a gun that would likely affect that specific attribute of the gun. The point being that once you do this, you won't help but start putting the same parts in multiple rows, and get a general idea of each different type of thing that particular part has an effect on. (You can even take the spreadsheet and flip the data, so you instead have each row listing a gun part, followed by a list of attributes that can be changed based on changing that one part, but I'd suggest building the spreadsheet starting with the attributes first, and then basing any new spreadsheets on that original data.)

Using that spreadsheet, you should be able to get a good idea of how the variables that control's the gun's activity should derive from the component's used to build the gun.

For instance, lets look at Ciox's example more closely. It would stand to reason that larger caliber guns would in general do more damage, and such an assumption works fine for a simplified firearms system we want to emulate. The problem is that the size of the caliber doesn't directly affect the number of shots you can fire, which is totally dependent on the size of your magazine. But we do know that larger caliber bullets are bigger, and therefore does affect how many bullets your magazine can hold. The point being that it is something that shouldn't be ignored when choosing a caliber for the bullets of your gun.

If we are to make a balanced system that gives the player freedom to customize, it stands to reason that the player should be able to take both of these factors into account. So we do it in a logically intuitive manner. Instead of giving each clip a variable that says how many shots fit within it, give it a variable saying how much area fits within the clip. This minor distinction makes all the difference, and works when you use the caliber of the gun to derive the size of each bullet. That way, when the game decides how many shots your custom gun can hold, it builds the number dynamically based on both the size of the caliber and the size of the magazine, instead of one or the other.