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APCP

APCP stands for "Ammonium Perchlorate Composite Propellant." APCP is the type of propellant used in many hobby rocket motors as well as other solid-rocket motors such as the ones in the Space Shuttle's Solid Rocket Boosters.

References:

Is APCP an Explosive? (Article)
Low Explosives Users Permit (Miscellaneous)
Ammonium Perchlorate Composite Propellant (Miscellaneous)
TRIPOLI ROCKETRY, et al v. BUREAU OF ALCOHOL (Miscellaneous)
Tripoli and the NAR filed a lawsuit against the BATFE over the ATF's classification of APCP as an explosive on February 11, 2000.
APCP Propellant Grain in Cardboard Liner (Picture)