I am repairing a Garrett sharps that from the factory had an over sized hole for the nipple and nearly no thread engagement. Drilled and rethreaded it for a 9x1mm nipple, however I didn't think to square the bottom of the seat, and don't think I want too much gap there to slow down the fire from the cap, does anyone have any expertise using a bushing to fill the gap below the nipple seat?

Any input is appreciated.