The short answer.... 20 LBS
The long answer.....
10-3 SMOKELESS PROPELLANTS. 10-3.1 Quantities of smokeless propellants not exceeding 25 LB (11.3kg), in shipping containers approved by the U.S. Department of Transportation, may be transported in a private vehicle.
10-3.2 Quantities of smokeless propellants exceeding 25 lb (11.3kg) but not exceeding 50 lb (22.7 kg), transported in a private vehicle, shall be transported in a portable magazine having wood walls of at least 1-inc. (25.4-mm) nominal thickness.
10-3.3 Transportation of more than 50 lb (22.7 kg) of smokeless propellants in a private vehicle is prohibited.
10-3.4 Commercial shipments of smokeless propellants in quantities not exceeding 100 lb. (45.4kg) are classified for transportation purposes as flammable solids when packaged according to U.S. Department of Transportation Hazardous Materials Regulations. (Title 49, Code of Federal Regulations, Park 173.197a), and shall be transported accordingly.
10-3.5 Commercial shipments of smokeless propellants exceeding 100 lb (45.4 kg) or not packaged in accordance with the regulations cited in 10-3.4 shall be transported according to the U. S. Department of Transportation regulations for Class B propellant explosives.
10-3.6 Smokeless propellants shall be stored in shipping containers specified by U. S. Department of Transportation Hazardous Materials Regulations.
10-3.7 Smokeless propellants intended for personal use in quantities not exceeding 20 lb (9.1 kg) may be stored in original containers in residences. Quantities exceeding 20 lb (9.1 kg), but not exceeding 50 lb (22.7 kg), may be stored in residences if kept in a wooden box or cabinet having walls or at least 1-inc. (25.4-mm) nominal thickness.
10-3.8 Not more than 20 lb (9.1 kg) of smokeless propellants, in containers of 1-lb (0.45-kg) maximum capacity, shall be displayed in commercial establishments.
10-3.9 Commercial stocks of smokeless propellants should be stored as follows:
(a) Quantities exceeding 20 lb (9.1 kg), but not exceeding 100 lb (45.4 kg), shall be stored in portable wooden boxes having walls of at least 1-in (25.4-mm) thickness.
(b) Quantities exceeding 100 lb (45.4 kg), but not exceeding 800 lb (363 kg), shall be stored in non portable storage cabinets having walls of at least 1-in (25.4-mm)thickness. Not more than 400 lb (181 kg) may be stored in any one cabinet and cabinets shall be separated by a distance of at least 25 ft (7.63 m) or by a fire partition having a fire resistance of at least 1 hour.
(c) Quantities exceeding 800 lb (363 kg), but not exceeding 5,000 lb (2268 kg), may be stored in a building if the following requirements are met:
1. The warehouse or storage room shall not be accessible to unauthorized personnel.
2. Smokeless propellant shall be stored in nonportable storage cabinets having wood walls at least 1 inc. (25.4 mm) thick and having shelves with no more than 3 ft (0.92 m) separation between shelves.
3. No more than 400 lb (181 kg) shall be stored in any one cabinet.
4. Cabinets shall be located against walls of the storage room or warehouse with at least 40 ft (12.2 m) between cabinets.
5. Separation between cabinets may be reduced to 250 ft (6.1m) if barricades twice the height of the cabinets are attached to the wall, midway between each cabinet. The barricades shall extend at least 10 ft (3m) outward, shall be firmly attached to the wall, and shall be constructed of 1/4-inc. (6.4-mm) boiler plate, 2-in. (51-mm) thick wood, brick, or concrete block.
6. Smokeless propellant shall be separated from materials classified by the U.S. Department of Transportation as flammable liquids, flammable solids, and oxidizing materials by a distance of 25 ft (7.63mm) or by a fire partition having a fire resistance of at least 1 hour.
7. The building shall be protected by an automatic sprinkler system installed accordingly to NFPA 13, Standard for the Installation of Sprinkler Systems..
(d) Smokeless propellants not stored according to (a), (b) and (c) above shall be stored in a Type 4 magazine constructed and located according to Chapter 6.
