For realism i would suggest the following.
Splitting guns into craftable categories: revolvers, semi/auto handguns, rifles. Each having components relevant to their category.
Revolver components:
Frame of set caliber and length, that can be recalibrated with the kits already ingame, but only to a higher caliber. Length can be cut down for weight reduction at the cost of accuracy, range and recoil. Multiple weights as well, that affect recoil and max caliber. IE: Medium frame that lowers recoil a bit and can be ported to .38 or .40. If wear and tear is implemented it could also affect overall durability based on caliber and type. (i was thinking of the frame and barrel in one revolver design, not the split design)
Cylinder of set caliber and weight, same as above. Note: with regard to durability: Cylinder should be the part of the gun that handles the most stress (since the bullet fires from here). Also a word from someone that knows the ins and outs of gun engineering would be helpful here, i’m no expert by any stretch of the imagination.
Mechanism: single or dual action, affecting accuracy and firing speed/recoil in the game. Also could be added to the firing mode selector.
Grip, mostly for bling. Various exotic materials like pearl handle, ivory and whatnot.
Handguns:
Barrel of set length, caliber and weight. Same as revolver frame.
Slide of set weight. Affects recoil and durability.
Frame of set weight and volume. Affects recoil durability and caliber. Max caliber determined by volume, cause you have to fit a mag in it.
Mechanism: semi and auto, auto being a mod of the semi (by removing a component if i understand it correctly, again someone with indepth gun knowledge needed)
Magazine of set caliber and capacity. Fits only one frame. For larger than standard mags, adds volume and weight.
Modifications possible to barrel: compensator, to decrease recoil; frame:
[ul][li]mag well, to decrease loading time[/li]
[li]stock, to increase accuracy and decrease recoil[/li][/ul]
; slide:
[ul][li]sights, affect accuracy[/li]
[li]porting, cuts weight and recoil, also makes the gun cycle faster, could be used to increase burst size in auto[/li][/ul]
Rifle:
Upper receiver: same as revolver frame
Lower receiver: same as handgun frame
Stock: modifies recoil and accuracy
Note: needs more detailed work based on rifle type: internal mag vs external, clasic vs bullpup, etc
Is this the kind of system you are looking for slax or is it a bit too involved?