Magnum rifle primers for T/C Black diamond
 

I am having misfires. I think I need a new breech plug. Should I go with the magnum rifle plug or back with the T/C replacement?

Have you cleaned the carbon from the flash channel? Once the carbon starts building up, you can start having misfires.

Take a 1/8" drill bit and turn it BY HAND into the flash channel. It will chip out the carbon buildup. Verify that no chips are in the flash hole. That's the first thing I'd check.

Ditto what Barnes said. Also, if you are talking about switching to something like a variflame plug, they really aren't needed. I was thinking myself about switching to a variant of them but figured that my overall results with my factory BP's was more than satisfactory. That carbon could be REALLY hard in your flash channel so keep at it until you get it all cleaned out. If you are using Black Horn powder, while yes it burns very clean and doesn't foul your barrel badly as other subs do, it is VERY bad about carbonizing your channel. I clean mine every 10 shots.

You-guys think the thread-starter Hook is shooting Blackhorn in his Black Diamond? Isn't that a plunger-action?

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Hook,

What makes you think your misfires are related to your plug and not your powder? We need more info, in order to help you.

Strange that the Black Diamond is having misfires. You can change out the nipple that screws into the breech plug. My Black Diamond is so sure fire, I use #11 nipple and CCI Magnum caps and have no misfires.

Could you tell what kind of powder you're shooting? If its Black Horn 209 I would suggest stop it. I had a primer blow back in my face out of a BD using that powder. If its other brands of powder and you're swabbing between shots, be careful how wet the swab patch is.

It misfires with both Pryodex pellets and BH209.

Sounds like your pellets either aged or have moisture contamination. Try new Pyrodex first.
If the problem persists after that and both ends of your breechplug are clear, buy a new breechplug and nipple.

Blackhorn is not recommended for non-209 ignitions (and/or) plunger actions.

My TC Black Diamond fires every time using #11 caps with Pyrodex, 777, Black MZ and blackpowder.

Pyrodex pellets have a blackpowder igniter on one end. That igniter should be facing the breech plug.

I doubt he needs a new plug, unless the plug and nipple are bonded together on that model. I'm not sure how T/C offered that particular plug.

Usually when a normal plug with screw-in #11 nipples age / get worn, the flash-hole at the powder-end eventually enlarges and more blowback thru the nipple hole occurs, right at the shooter's face.

It's probably those pellets at fault here, plus the Blackhorn powder improper/misuse. Only other answer would be if that plug and #11 nipple are made bonded together.

The Black Diamond breech plug is threaded for nipples that accept either #11 caps, musket caps or 209 primers.