Looks like a Traditions breach plug. The way the plug is made I had all kinds of fouling problems until it was bored out to cal. dia. I beleive the threads are meteric as is the nipple thread. M 6 X 1. Check the bore for any tight or loose areas . The little .32's shot pretty good we use them at the National Wild Turkey Federations "JAKE " event where there is over 450 kids shooting these riflews.