The mainspring, or hammer spring, and the frizzen spring need to be in proper balance. The frizzen spring, especially, needs to be strong enough to resist opening the frizzen too easily. A frizzen spring too much weaker than the hammer spring will allow the frizzen to pop open, flying completely away from the flint as it falls. The flint needs to scrape down the face of the frizzen, or it will not cut steel and spark well. Most any name-brand maker of locks in this country produces locks in which these factors have been worked out, and their locks can be depended upon.
Locks are available in three basic sizes, musket, rifle and pistol.
Black British flints are highly thought of, today, and many shooters prefer them. The main type used instead was the tan colored French flints, apparently the standard of the time. Today, either will work well. All those flints were made by knapping them from a large spall of flint. Also available today are sawed flints, cut from agate or other stone.