What I am trying to do:
Import a model from blender into Garry's Mod (a video game). I applied a basic skeleton with the proper format needed to port a model into the game:
When I fixed the model to port into the game, I used a strict skeleton implemented in most player models in the game, so the armature for the model is not custom and I was forced to copy and paste to maintain the games format.
The model looks fine in a 't-pose', the pose it was rigged on in blender, the one pictured above or, in game, below:
When the model attempts to run animations like holding a weapon, the textures stretch: