Are these the flints that are sawn, instead of knapped?
If so, you're probably not going to have much success with them at all. I never did.
I invested in some some English flint (or chert) at a gunshow. About $4.50 a flint
but WELL worth it. All told I got nearly 100 shots out of the one before I had to scrap it.
One of the tricks you'll need to learn is using your short starter to "field dress" the flint while it is in the cock. Mine has a brass stub on the end that makes a VERY nice knapping tool.
With some practice you'll be able to sharpen your flints in no time.
Another thing I've found is that the flints ABSOLUTELY have to remain in the jaws without slipping. If the flint starts to slip forward every time the cock drops, it's going to shatter sooner or later.
I never had much success in using leather to hold the flint in place. I finally got frustrated one day and, remembering something I read in a book a long time ago, I pounded two roundballs flat and wrapped the flint in them, then cranked down the jaws. The flint stayed EXACTLY where I put it until I took it out again.
What model flintlock do you have? I built my T/C Renegade from a kit. It was a LOT of fun, but I finally had to give it over to the gunsmith to finish it. I couldn't get an even brown on the parts. All the different steels took the brown at a different rate, so I finally had it hot tank browned.
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Beware the man with the S&W .357 Mag.
Chances are he knows how to use it.