This is a very old bug connected to vehicles / weapons / backpacks
Weapons with long barrels when in a shoulder slot goes through the roof when player seats in vehicle (especially when improvised suppressor is attached). Almost all backpacks goes through front seat.
There is very simple and elegant solution: hide shoulder/back slots while player is in vehicle. I saw that script few years ago, but I was to stupid not to copy it. This is minor bug, and with time became kinda feature, but this is not cool infact. If you aren't going to fix this in nearest future at least drop links for side mode (if you know one) that fixes it, or just drop a script for that as an open source. Also, ANYONE who saw that script or know how to write it, I'll be greatful for providing one. I almost gave up trying to find one.