Help! Barrel nut goes 1/4 turn past the gas tube hole

[RBruce]

I removed the FF tube. Cleaned and reapplied moly to the threads. Tightened, untightened then retorqued the tube four times. I can get the gas tub slot only to the six o'clock position. What am I doing wrong? Or, do I need some shims? **Note: this is a FF tube that has standard A2 handguards (no heat shields) so it looks like a normal issue A2 for CMP matches.

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