MY Opinion: Both the procussion and pressure flaking techniques require that some of the force of the blow or pressure be absorbed by the billet (stone, horn, etc) or horn flaker, copper, bone, while the remainder of the force is imparted into the material and hopefully at the desired angle. Copper has been around in some areas for thousands of years, just not in the refined and shapes we now have. The copper you can buy at the hardware store is soft drawn and although it will grab on to the abraded area of your material it will bend and deform and is often not tough enough to force the desired flake off. Since iron, steel, brass are hard they tend to crush rather than flake as there is no real amount of force returned to them when a flake attempt is tried. Horse shoe nails are used primarily for notching and edge finishing where you are only attempting small prescribed flakes. Cross country power lines use hard drawn copper wire as well as deep well drillers often use it for down the hole current! You can change the hardness of soft drawn copper by twisting, stretching as with copper tubing, or pounding the ends on a hard surface. The copper wire used in the old days on power poles for ground wire is often hard drawn and is an excellent source of good useable copper. In the 50s I used lots of antler, horn, fire harden wood and stone and all were readily available on the ranch where I lived. Now that I am old and have lost a lot of wrist strength, I have gone to copper ended Ishi sticks for the advantage they provide. Any way you can carve out a useable blade be it a small projectile or a big skinner is very satisfying and I hope you find this knapping journey one of obsession and pleasure. A/Ho Pokie