Because some number of people do have trouble with this when making guns.
You're here because none of the pre-defined(Hard Coded) Reload Animations in EnumAnimationType.java for your gun doesn't seem to work so well for your model.
So then there's CUSTOM, which is a blank reload animation, allowing modellers to design their own.
To use it, there is a set of parameters in ModelGun.java that affect the values it uses to create the animation.
These parameters can be mixed and matched together to build the animation. It may take a lot of trial and error to get them right.
Any parameters that you do not use in your model file will simply be set to 0.
(I'm assuming you know your way in and out of these model.java files. You will need Notepad or something similar to modify the code when needed.)
Winnow AP reload code in the Greylight Pack. The code as follows, will rotate the entire gun horizontally by 60 degrees, and translateClip will 'eject and insert' the clip by 4.
GSX Pistol reload. The gun is rotated/angled upwards by 10 degrees and as ammo clip is swapped out, it is moved by 3 and additionally angled by 5 degrees.
-Note that these parameters will only ever work with the CUSTOM reload, and none of the Pre-defined ones.
-The translate moves the model in the order of (x, y, z) co-ordinates.