measure the width of your frizzen . this will be the proper width of the needed flint.
Length of the flint depends on the lock and its geometry . This is why you see square and rectangle flints often listed .
IE with a long fall lock , you will never find a flint that will set just of the frizzen face while at half cock
with long fall locks the recangle flints normaly work better . while short fall locks normaly like the square flints . but as with all things in muzzleloading , the are exceptions .
Depending on who you get your flints from , rectangle flints can be rather long . But you can notch the back of the flint so as to set around the cock screw
Same goes for the so called square flints. Normally they aren’t truly square
When it comes to flat top flints like those Sir Michael shows in his link to ToW .
Know that these very in thickness and may not fit your lock even though the width matches the frizzen and the length is right
Right now I have a Early Davis lock that will not accept about ½ of the 1 inch Fuller flints in my stock pile .
Its not a real big issue and its easily remedied by shortening the length of the cock screw by a couple threads or using thicker padding to the jaws .
but if i used a peaked top flint . its will set just fine with no issues
The other thing to keep in mind is that all flints are not good for all rifles . For that mater are all flints good .
Concerning the first part of this statement . Some times a lock will simply eat up English flints . But changing to a French amber will greatly reduce this problem . Same with quality of spark . Some times a lock just prefers ambers of English flint.
My Hershel rifle is that way . For years I used English flints . The lock would give me around 75 falls to a flint and it sparked well enough . But after switching to ambers , I now get near 100 + falls per amber and a more consistent spark .
The opposite holds true for my SXS . The locks she holds , prefers English flints in that they give better spark.
As to the second part of the statement .
One of the reasons I stopped ordering flints is that I was finding “concerning those from ToW and Dixie “ about 25% of those I would receive “buying in bulk “ would be IMO poor quality . I will say this though . The last batch of 500 flints I got directly from fuller only had maybe 2% that I regret paying for .
As such today , I prefer to pick my own flints. As such when im at an even and there is a trader selling flints , I pick through and chose my flints carefully, looking for consistent color , shape and thickness .
there is no use paying for a flint that will break the first time you Knapp it or shater after a few falls .