I must have cross threaded the nipple port in the breach of my .45 cal. Pedersoli Tryon target rifle.
I'm not sure what the thread is on the nipple of this rifle. I have owned it for 25 years and never had any issue till a few days ago.

What is the best fix? Take it to a gunsmith or try to clean up the threads with a tap. The nipple comes out of the breach at an angle so to retap is a little challenging.

Metric, probably 6x.75mm. Be careful; might be able to make it a 1/4x28 if you mess it up. Next time, never horse a nipple into place , but you know that.

I would take the nipple to a hardware store and determine what the thread and pitch are, for starters. Then buy a bottom tap of the same pitch and very carefully chase the threads. If you are lucky they will clean up.

The other thing I would recommend is Teflon tape or similar when you screw the nipple in...

Contact Dixie Gun Works. They have oversized nipples made in increments of two or three thousandths that are a harder steel than the standard nipple. You can chase the threads with these and find one that will be a good fit. These nipples are not expensive, and work like a charm. They are in fact an old gunsmithing trick from the 1800's. BTDT.