Having issues with animating my custom l4d2 character

I’m trying to make a custom skin for Left 4 Dead 2 in which Nick is replaced with Booker Dewitt from Bioshock Infinite. I’ve successfully ported the mesh and textures from the original game, rigged the skeleton, and weight painted for the vertices. The model looks fine when no animations are being applied, but as soon as I try to animate the model, it gets all distorted in strange ways.

How the model looks during animation in HLMV

How it looks in Blender

As far as I can tell, it seems like my new model is trying to conform to the old Nick model (Booker’s arms are longer than Nick’s, and most of the distortions are in the arms, while the body is perfectly fine except the left toe). Has anyone else encountered this problem? Any ideas on how to fix it? This is my first reskin mod for Left 4 Dead 2 so this is all rather new to me. Any help is greatly appreciated and you will get credit for it.

EDIT: I have tried applying rotation, scale, and location and I’ve also tried selecting the “preserve volume” option under the armature modifier.