The black english flints work well for me.I have the flint make as much spark as possible without touching the frizzen at halfcock. Bevel up or bevel down doesn't matter to me as long as I am getting good spark.
I use flints from Rich Pierce over on another forum. They are white, I believe they are referred to as chert. Your flint should strike about 2/3rds of the way up your frizzen, be it bevel up or bevel down. I like to keep mine just a fraction of an inch off the frizzen at half cock.
Seems to depend on the rifle when it comes to cut agates. My GPR hates everything but cuts. I get numerous shots from one end and then flip for a tons more. A fella onec told me it depends alot on the lock/hammer angle etc. etc.. I just played around and found what worked for me.
It can't be touching at half cock as that will lift the frizzen and the powder will spill out, potentially dangerous also if the frizzen can rub the flint, a very small gap should be seen equally across.
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