Now at a crisis

[RetiredUSNChief]

I tried a 5/16 x 20 nipple. It didn't work. Here is the pic of my gun's nipple on left and the 5/16 x 20 on right. I am thinking either a 5/16 x 18 or 8-1.25 mm. Any suggestions?

Yep.

Thread gauge. $5 at your local hardware store.

Or go to Sears withb the nipple and try some dies until you find the right threads.

Then you'll have your answer for sure!

 

[Dave Markowitz]

FYI, if you have a nipple that looks like it's made out of brass, it is almost certainly an Ampco nipple, which is a special corrosion-resistant bronze alloy.

 

[desidog]

If it's a Parker Hale -made musketoon it should be 5/16-18 thread.
&
If it's an Italian-made (Euro-arms or Armisport) it should be 8-1.25mm thread.