On my AR. I'm changing the forend and it comes with its own barrel nut. When I put the new one on, it screws all the way up to the receiver and stops. There is still a bit of wiggle to the barrel. I didn't try and torque it because it doesn't look like it can tighten anymore. Upon inspection it seems the newer nut is slightly deeper. This is my first time to do this and I didn't know there where different depths for the nut. Is there some kind of spacer washer that goes in it? Whats wrong here?