Re: Blackhorn 209 new black powder
1/4 od x 1/8 id x 1/16 thick. Lowes #60 in plumbing. It's for faucets so I assume it will stand up to higher temps. I just asked the H&R guys if they came up with anything better. I will let you know.
All guns have different head space and all brands of primers are different heights. You want a 20% compression fit. Take your plug with your primer of choice (recommend cci 209M) and use a caliper to measure the total stack. Deepen it by 1/32(half o-ring), remeasure stack often and try it. Go slooow!! Your guns headspace may be larger and if you go to far it won't fire. I used a finger drill with a new 15/64 bit. Lowes has several brands, I choose one with flattest point. I would start with an old plug with a clean exit hole. Exit hole can be up to .004.
I'm sure Home Depot or your local hardware has them too. Lowes is just closer for me.