If cannon fuse is considered safety fuse, it's mailable as has been stated, safety fuse is the only fuse that is mailable, all other fuse types are considered a nonmailable explosive.
341.2 Mailability, Packaging, and Marking
Explosives are generally prohibited from mailing under 18 U.S.C. 1716. The following conditions apply to the mailing of explosives:
1. International Mail. All explosives are prohibited.
2. Domestic Mail via Air Transportation. All explosives are prohibited. 3. Domestic Mail via Surface Transportation. Generally, explosives are prohibited. The only exceptions are for Division 1.4S toy propellant devices and safety fuses that have been approved by the Manager, Mailing Standards, USPS Headquarters, Washington, DC prior to mailing as stated in 341.22 and DMM 601.10.11.2.
341.21 Nonmailable Explosives
Nonmailable explosives found in the mailstream must be immediately reported in accordance with POM 139.117.
Nonmailable explosives include, but are not limited to, the following:
1. Common Fireworks. Fireworks are classified as Division 1.1, 1.2, 1.3, or 1.4 explosives depending on the degree of hazard. Fireworks include roman candles, skyrockets, helicopter–type rockets, cylindrical and cone fountains, pyrotechnic wheels, illuminating torches, firecrackers, salutes, and combinations of items that are designed to produce any of the aforementioned types of effects. All types of fireworks are prohibited from mailing. 2. Fuses. Fuses are classified as Division 1.3 or 1.4 explosives depending on the degree of hazard. All types of fuses (except safety fuses as permitted under 341.22) are prohibited from mailing.
341.22 Mailable Explosives
The following specific types of explosives may be mailed only when the applicable conditions are met. Full responsibility rests with the mailer to comply with DOT and Bureau of Alcohol, Tobacco, and Firearms (ATF) regulations before mailing.
1. Toy Propellant Devices. The proper shipping name for a toy propellant device is “model rocket motor” or “igniters.” A toy propellant device assigned UN0454 or NA0323 and classed as a Division 1.4S explosive is eligible for mailing in domestic mail via surface transportation only when prior written permission has been obtained from the Manager, Mailing Standards, USPS Headquarters, Washington, DC. A device approved for mailing is subject to the following conditions:
1. Each device must be ignitable by electrical means only; contain no more than 30 g (1.07 oz) of propellant; and produce less than 80 newton seconds of total impulse with thrust duration not less than 0.050 second.
2. Each device must be constructed so that all chemical ingredients are preloaded into a cylindrical paper or similarly constructed nonmetallic tube that does not fragment into sharp, hard pieces; must be designed so that it will not burst under normal conditions of use; must be incapable of spontaneous ignition under 500° F; and must not contain any type of explosive or pyrotechnic warhead other than a small, activation–charge, parachute–recovery system.
3. Each mailpiece containing approved devices must be prepared for mailing following Packaging Instruction 1A in Appendix C. A shipper’s declaration for dangerous goods is required. 2. Safety Fuses (UN0105). Safety fuses consist of a core of black powder over–spun with yarns, waterproofing compounds, and/or tapes. Safety fuses assigned UN0105 as a Division 1.4S explosive may be mailed in domestic mail via surface transportation only when prior written approval has been granted by the Manager, Mailing Standards, USPS Headquarters, Washington, DC. Mailable safety fuses must be prepared using Packaging Instruction 1B in Appendix C. A shipper’s declaration for dangerous goods is required.
Note: “Safety Fuses” are not to be confused with “fusees.” Fusees are rail and highway distress signals that are nonmailable Class 4 flammable solids.