Bodygroups
Bodygroups are what controlls which parts are currently shown, or not shown on a gun. These are used to change things, all from handguards, to stocks, to what attachments the gun is equibed with.
This will not be a full list of all bodygroups, as there is simply too much to cover. I will pick a few specific ones, that i will explain below.

Matching parts
Often when changing to a shorter, or longer barrel, the frontsight and rails may become unfit.
To figure out which part fits with the handguard you've choosen, simply look at the name, and find one similair in the corresponding group.
Example for M16:
Let's say we choose Handguard04_JPRS, as can be seen the name has an S at the end, this stands for "Short" to get the correct frontsight for this piece, we will choose Frontsight01_M16JPRS, as can be seen, this name implies that it is both for M16 and JPR "Short" handguards.
For the side rail, we will go for RailSide_Short, again the name has "Short" in it, meaning that it is for "Short" handguards.

If two pieces go together, their names will usually imply this. Suppressor_M16 goes together with Handguard02_M16
Keep this in mind, and you will save time looking through the bodygroups!

NOTICE if a bodygroup part says Model(number)# it will 'hide' this piece.

Common Bodygroups
Handguard - Contains different handguards, barrels length change with the handguard.
DustCover - This toggles a Dust Cover over the ejection port.
TopRails/BottomRails/UnderRails/sideRails - This Toggles rails on the corresponding sides.
Magazine - Different or empty magazines. E at the end of names, means Empty
Attachments/TopAttachments - Attachments ontop of weapons.
TacAttachments - Toggle between different tactical equibment, i.e. AN-Peq, flashlight, and bipods.
UnderAttachments - Toggle between different underbarrel equibment, i.e. M203, Masterkey, and vertgrips.
One_In_The_Chamber - Toggles a visible bullet, if the slide is pulled back.

Mossberg
Magtube - Change between different lenghts of magazine tubes.
Connector - Barrel guide rings.
870MCSRailCarrier - Additional rail for the 870MCS barrel
Muzzle - Adds different breaks, or suppressors to the muzzle of the gun's barrel.
GripAttachments - Attachments for the pump grip, R = Right. L = Left. "Under_M3S90Taclight" is an integrated Flashlight for the M4 Benelli pump grip.

Neostead
CarryHandle - Changes the usual carry handle appearence. Use "CarryHandle_Railed" for optics.

PX4
Compact_Frame - If toggled, changes to a smaller frame.
Compact_barrel - If toggled, changes to a smaller slide and barrel.
Slide_release_up - Quick way to set the slide catch 'up'
Optics_rail - Toggles an optic rail, this is attached using the lower front rail. Used for serval attachments.
Rail_Right/Left/Bottom/Top - These are used to add attachments to their respective 'position'

S&W M1911
Grip_Infinity - Toggles SVI Infinity styled grip
RecoilSpring_Rod - Plunge is replaced with guide rod
Front_sight_dot - Toggles type of front sight 'dot' Ironsight_Dot_Infinity is for the Infinity slide
Addon_Griprest - Toggles bulgy handrest on grip

Kriss Vector
ChargingHandle_TypeB - Toggles a non-prototype cocking lever