Diagram of British No. 15 Base Percussion Fuze, Large, Mk III.
Made of aluminium bronze, and is stronger than the otherwise identical No 11 Mk V, so that there is less chance of the fuze being blown out and failing to cause the shell to burst.
For use with all capped A.P. shell and capped common-pointed shell. May be used with uncapped A.P. and uncapped common-pointed shell.
This diagram also identifies No 11 Mk V Base Percussion Fuze. It is identical to No 15 Mk III except that the body is made of Class "A" metal.
Action :

• On firing, gas pressure is exerted on the pressure plate at the base, driving up the plunger (coloured red).
• The plunger while at rest restrains the centrigual bolt (oposite point A), and when the plunger moves up it ceases to restain the centrifugal bolt.
• The shell's spin causes the centrigal bolt to spin outwards towards point A, pushing the small retaining bolt into the recess in the fuze body opposite point A.
• This opens up the flash channel through the centre of the fuze, and also frees the bolt from the recess in the fuze body opposite point B. At this point the fuze is armed, and ready to ignite when it strikes a target.
• When the shell strikes a target, it decelerates and the inertia of the central body, which is now unrestrained, causes it to continue moving forward and impales the detonator cap onto the steel needle.
• The detonator compound ignites and sends a flash back down through the centre cavity, through a passageway now opened up into the passage (coloured black) filled with perforated powder pellets which leads to the head of the fuze.
• These pellets burn, providing a time delay allowing the shell to penetrate the target, until the flame reaches the compressed powder ring at the head of the fuze, which is deep inside the shell, and hence ignites the shell's explosive filling.