12g Pierce Pin

These are not stainless, so light surface corrosion may occur over time, and discoloration from the heat treat process is normal. 

To keep costs reasonable, these will ship with common hardware washers, which work, but require a break-in run. By firmly tightening a spent 12g to the pierce several times, the polymer washer will form to the cup and create a good seal. No sense wasting a fresh one on that first "set", though. 

Since everyone wants a better mousetrap, the seat specification is 5/16" OD, 1/16" thick, and .125" or greater ID. Let me know if you find a material that's just right - a 90A urethane is looking like a front-runner if the right size is available.