@Hibbey - cheers for the feedback and sorry about your ears!
I'll do my best to make this brief. The demo is very very very outdated - it has since been re designed and created from the ground up using a completely different approach which is more efficient.
There is no ammo display in the newer versions and the R+E combination has been changed to a single button release (too many people found it annoying)
In terms of customisation, believe me, the issues you raise are exactly the same questions I asked myself and I tasked myself at solving but when you take a while and think about it they are not as complex as you might think.
The whole system uses base statistics for weapon parts, take for example the Beretta - it has benefits over a Glock but both are equally balanced in performance. Each component has a performance weighting (barrel, magazine, overall weight, interoperability). An example of this in more detail is that the barrel of the Beretta allows the round to leave the muzzle at 0.011 and the Glock projects at 0.010 the balance comes in when you take the weight of the component into account - the Beretta barrel weighs more than the Glock barrel and contributes negatively to maneuverability resulting in a slower push-out to get a proper sight picture. This balancing act is performed with all components.
It is up to the player to choose if he wants to use the Glock barrel in their build or a Beretta barrel (Both operate with the 9mm cartridge, but differ in length statistically), the performance modifiers of the weapons can change but there is always balance, changing the geometry of the shorter barrel to make it longer will have no statistical impact on the weapons performance - the combination on how it is employed will.
The customisation never changes the base statistics of the weapons. Using different components together changes the overall performance of the weapon but by using certain parts you also make sacrifices. Trying to use a .45 cartridge with a 9mm Barrel is not going to benefit you much.
If the player decides to create a build using all of the best components their weapon will quickly malfunction. Weapon component customisation is a high-wire act. Weapon cosmetic customisation isn't.
Changing the geometry for the weapons is no more than a cosmetic change. I'm sure some people will want to create
something like this...
Where it gets complicated is making sure that players can't create weapon components that are the size of cars by loading in for example... a CAR! Bounds are used for that. They will have a really funny looking gun though! and yes there are going to be some stupid looking guns. There are 'pure' servers to combat that.
There is a lot of balancing involved, too much to go into real detail about. I'll release the statistical information on the components after launch.
Cheers again!