Your figure of 100 million guns is way off I believe. The number is more in the order of 300-500 million, and that is only the legal ones. No one really knows because there were no records kept until the last part of the 20th century.
Quite right. The other fact they don't get is that there are these people called machinists, who make things out of metal for a living, and could use their skills to make pretty much any type of weapon they want.