A good magazine-size paperback book by D.C. Waldorf, "Art of Flintknappin" is an excellent book to get started with. At the very end of his book, he gets into how gun flints are made and they are made quite differently than arrowheads, spearheads, knives, and axes. You really need a core to strike off long flint shards and then you can get two or three gun flints out of each shard. He also had a video out demonstrating many of the things in his book at his cabin in the boonies with a bird that would often come walk around on the table while he was knapping.
I'd also recommend buying a starter kit. It should have tools to do both percussion and pressure flaking. You'll also want about a 12" square of 9-oz. leather (thick) to drape over your knee when percussion knapping and a smaller piece of leather that you can fold into about a 1/2" leather pad or a slotted rubber hand pad for doing pressure flaking in your hand. And, so you don't go through tons of bandaids, get some nitrile gloves that are thin enough to let you feel what's going on but strong enough to protect your fingers from cuts. Be aware that flint shards are extremely sharp, especially obsidian.
There are basically two approaches to flintknapping. One is known as "Abo knapping", which is short of aborigine tools flint knapping. Abo tools include deer antler tines for pressure flaking and either the root of an antler that has been rounded off or round-edged river rocks for percussive flaking. The advantages of using antler is that they tend to throw longer flakes because the compress slightly as they hit the flint and then spring back as the flake starts to release. Using a rock to flake doesn't throw as long of a flake because a rock doesn't compress and then release extra pressure link and antler will.
Round edge River Rock for Percussion Spawling and flaking depending upon size.
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Deer, elk, or moose antler root billet for percussion flaking (use rounded end)
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Deer, elk or moose antler tine pressure flakers.
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The other way to knap flint is by using modern flintknapping tools. Modern flakers typically use a copper welding rod that has been filed or sanded to a point or a copper nail that is embedded in wood or Delrin. These don't throw as long of a flake as antler tips will but rarely need sharpening and are very consistent in their results.
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Instead of an antler billet for pressure flaking they use "copper boppers" which come in all sizes from about ¾" to 2" in diameter. These have dome-shaped copper endcaps with lead inside the copper cap for weight.
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Google "flintknapping near me" (or near
your city) to see if there's anything going on near you. I found a spot about 35-minutes away where they have regular "knap-ins" and that was very helpful.