Austin Halleck flash hole thread size?

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I have a new to me Austin Halleck Mountain rifle. Does anyone know what size flash hole liner it takes?

 

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I contacted TOW and they knew the vent size I need-5-.8mm. They are out of stock right now.
The gun is new to me and I haven't shot it yet.To look at it the vent liner appears to be new. My concern is that I don't want it to get stuck. I went through my screw drivers until I found one that fit. At first the liner won't turn at all, finally it turned about an 8th of a turn then the screw head started to be damaged. I stopped and turned the liner back to original position. I am not sure if I should leave it until I really know there's a problem or if I should find a way of removing it and properly grease the new one? Any thoughts?