I've never forgotten the captured NVA film we were shown at the School of Military Engineering in Casula, NSW, in 1970.
This film recorded an NVA sapper finding an unexploded U.S. bomb, defuzing it - then cutting around 7/8ths of the circumference of the casing with a hacksaw, leaving around 1/8" to 1/16" (3 - 1.6mm) casing thickness - then tapping the bomb with a sledgehammer to break it apart, so the NVA sappers could dig out the explosive by hand to fabricate crude booby traps, mines and hand grenades of their own.
These blokes even had little jungle foundries going, smelting the U.S. bomb casings to form them into their own hand grenades, using their own grenade casting molds!
The incredible part about the NVA effort is understanding that the U.S. bombs not only had anti-withdrawal mechanisms designed to kill the untrained person trying to withdraw the fuze - they also often had time-delay fuzes, activated by slow-working chemicals, that would also lead to "failed" bombs unexpectedly going off, up to 144 hrs after they were dropped!
Allied forces developed their own designs of anti-handling devices during World War II. For example, the American M123A1, M124A1, M125 and M131 series of chemical long delay tail-fuzes which were used in air-dropped bombs, starting around late 1942 and remaining in service until the 1960s.
Frequently fitted to M64 (500 lb), M65 (1000 lb) and M66 (2000 lb) general-purpose bombs, these fuzes were primarily designed to operate as chemical long-delay fuzes, with settings ranging between 15 minutes and 144 hours.
The time delay mechanism was simple but effective: after being dropped from the aircraft a small propellor at the rear of the bomb revolved, gradually screwing a metal rod into the fuze, crushing an ampoule of acetone solvent contained within it.
When this happened the fuze was fully armed and the timer countdown had started. The acetone soaked into an absorbent pad next to a celluloid disk which held back a spring-loaded firing pin from a percussion cap connected to an adjacent detonator.
Acetone slowly dissolved the celluloid disk, gradually weakening it until the cocked firing pin was released and the bomb detonated.
The time delay of the fuze varied according to the acetone concentration and the thickness of the celluloid disk.
Removing a chemical long delay fuze from a bomb after it had been dropped would have been a straightforward process had it not been for the fact that there was an integral anti-withdrawal mechanism designed to kill anyone who tried to render the bomb safe.
Fuzes such as the M123 (and its derivatives) contained two small ball-bearings at the lower end which slid out of recesses when the fuze was screwed into the bomb by aircraft armorers.
The ball-bearings jammed into the screw-threads inside the fuze well, preventing the fuze from being removed. Because the lower end of the fuze was locked in place deep inside the bomb (where it was hard get at) this posed major problems for enemy EOD personnel.
Attempting to unscrew a fully armed chemical long-delay fuze caused it to split into two separate fuze assemblies.
This action automatically triggered detonation by releasing the cocked firing pin in the lower fuze assembly, with lethal results for anyone nearby.