USPS Shippable Motors
Motors with 30 grams of propellant or less per grain may ship inexpensively via USPS Parcel Post, avoiding the expensive HAZMAT shipping fees associated with larger propellant grains. Most of these USPS shippable motors are smaller black powder motors in the A - D range. But with the higher specific impulse found in ammonium perchlorate composite motor propellants, composite motors can pack a lot more impulse into that limited propellant weight. Additionally, creative manufacturers package some reloadable propellant grains individually within the reload package, allowing the motor to comply with dangerous goods shipping regulations, even thought the total propellant mass for the combined grains is far more than 30 grams. So you will see a few reloadable motors in the G, H, I, and J impulse classes that still qualify for USPS ground shipping!