I think with most locks you can use 3F. About 20 years ago someone told me 3F would be ok in a bind, if you crushed it some--you don't need to. My Pedersoli Kentucky has a 1/2 flint--have only used 4f. I have a small Siler with a 5/8 flint, and a Robbins with a 3/4 flint--used in matches and primed with 4F from a priming horn. But Shemanese, my early Viginia from Narragannsett, has a Chambers lock, with a 7/8 flint--I tried 2F in the pan and it worked wonderfully. When she and I hit the woods, she is primed by 3F form my main horn--a priming horn would not be historically correct in a F&I portrayal, I believe. Keep yer powder dry, yer flint sharp, yer nose to the wind, and an eye on yer backtrail.