Explosives are classified according to their properties for purposes of shipping. The basic classification is a two digit number and describes the basic hazard.
| United Nations Explosive Hazard Codes |
|---|
| 1.1 Substances with a mass explosion hazard 1.2 Substances which present a projection hazard but no mass explosion hazard 1.3 Substances which present both a fire hazard and a minor blast or projection hazard (or both) but not a mass explosion hazard 1.4 No significant hazard 1.5 Very insensitive substances with a mass explosion hazard 1.6 Very insensitive articles with no mass explosion hazard |
Before any explosive material can be shipped, it must be classified by the Department of Transportation (DOT) so that its hazards are known. This process can be quite expensive. This site provides a quick background on the process. For more details than you probably want, look through the UN classification procedures. I don't know which specific test causes APCP to be classified as an explosive, but I suspect it is simply that it is designed to produce an "explosive or pyrotechnic effect". Which include such mundane things as producing heat, smoke, or fire. Test 6(c) is the one which decides which division it is in. Since APCP burns really well, this test puts it into division 1.3.
Just because something is classified by the DOT with an explosive hazard class, does not mean that it goes "bang". For example, road and aerial flares are classified as 1.4 explosives. (See Material Safety Data Sheets for Orion flares.)
Because the materials used in hobby rocket motors are hazardous materials (except for a few hybrid motors), shipping is complicated. For the most part only trained and certified shippers can ship HAZMAT. This prevents the average rocketeer from shipping motors legally.
But you can transport your own motors to the launch site because the DOT exempts private non-commercial transport:
49 CFR 171.1(d) Functions not subject to the requirements of the HMR. The following are examples of activities to which the HMR do not apply:
(6) Transportation of a hazardous material by an individual for non- commercial purposes in a private motor vehicle, including a leased or rented motor vehicle.
Some smaller motors can be shipped via USPS but even this requires prior authorization.