Make sure that after you edit the weapon file, you build your mod in Launcher.
Instead of putting (solved) in the title mark your post as the best answer so that it has a green border.
If you have the disc version of WAW, you need to have the LANFixed.exe along with the .dll file.
When the error pops up, press the key below your ESC key and your SHIFT key at the same time. In the console it should display a more detailed error. Tell us what it says.
I don’t think there’s a stock rechargeable mountable LMG but adding bipeds and mounting animations is possible. Just follow everything on the UGX Wiki tutorial and it will work.