Ah yes, the Customized AR-15.
So yeah, this is an AR-15, with several customized parts and attachments, made by Craig Collins on Artstation, which you can see here:
https://www.artstation.com/artwork/Jz8Xn

Unfortunately the original model does not have any bones, since it was not rigged, oof.

This model was ported from SFM workshop with permission granted:
https://steamcommunity.com/sharedfiles/filedetails/?id=2176571686

Includes:
ar15\ar15.mdl

Credits :
Craig Collins[www.artstation.com] - Original creator of the model.
JuVe913 (Divined) - Original creator of the SFM addon.
NeoLin - Porting to Garry’s Mod.