Briefly, google KM and look at any of the KM associations in the "about us" or "history" sections. You will get a pretty consistent explanation of any KM history prior to the death of Imi Lichtenfeld. KM splintered into many organizations around the world after that.
KM is taught at different levels. Military, Law Enforcement and Civilian Self Defense. Obviously, a different level of use of force is necessary for each group. But in the simplicity of technique it is easy to see how punches and elbows flow into the same movements such as using an umbrella as an elongated weapon and finally to using a rifle or machine gun as a blunt weapon. KM weapons techniques are drived from the Israeli military's use of instinctual shooting.
why don't we use guns and knives as the basis?--We live in a society with laws and rules and try to be good citizens. We learn it is best not to fight. But, if someone wants to fight then we are taught to "overwhelm" the criminal or terrorist. We must not forget that the Israelis live with terrorism every day and that here in the US we only here about a small percentage of bombings and we never hear about the attacks that have been prevented.
So that we may walk in Peace.