S&W Mod. 3000 Shotgun

I'm having a problem with one of these...

When barrel nut is tightened all the way action won't stay locked and trigger won't function. Loosen up on it a bit and it works. Now that just can't be right! Barrel actually wiggles a little when loosened enough to function.

Anyone have any tips? Please e-mail if you do.

Thanks.