I've always used hand-knapped black English flints, 3/4 wide x 7/8 long, but since prices have gone up, I've been looking for alternatives. I bought a dozen hand-knapped "red Texas" 3/4 wide x 7/8 long flints to try. My lock is a deluxe Siler in a rifle I shoot matches and hunt with regularly for the past 5 or 6 years.
I installed one of the Texas flints and dry-fired the lock. The frizzen opened about 1/4" and the flint hung up on the frizzen, as if the flint was too short. I tried another of the Texas flints with the same result. Then I installed the same sized English flint and she operated perfectly, shower of sparks, frizzen snapped open smartly.
I'm at a loss. I repeated the test several times, making sure the flints were the same dimensions, mounted the same, etc. Could it be the Texas flints are harder/softer, sharper/duller than the English causing them to "bite" into the frizzen?
-Ron