I have a Green Mountain 15/16" .54 percussion drop-in barrel that I want to convert to flint and am concerned as to what size the threads will be for the plug. I know that their current .54 15/16" barrel blanks are 5/8 x 18 and wonder if they would be the same size or not. Has anyone removed a plug on a drop-in or know for sure what thread size it would be?