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It is most likely not a headspace problem, Shotgun shells are rimmed and "headspace on the rim" unlike most rimless handgun cartridges that headspace on the end of the case, or on bottleneck cartridges that headspace on the shoulder. Anyhow, if loosening the barrel nut allows the bolt to lock up and the trigger to be pulled, then I would examine the gas system parts very well, something is pushing back against the bolt when you tighten the barrel nut and keeping it from locking up. On a separate topic, there is a plastic grommet in the bolt that surrounds the firing pin. This grommet " mashes out " toward the front of the bolt face over time, and eventually will cause misfires, or light primer strikes. I have had to disassemble the bolt in the field before and shave this grommet with a pocket knife to get the gun firing again. I ultimately replaced it and resolved the problem. Anyhow this problem will not cause your gun to not lock up, it will just cause light primer strikes.