Hi Frank,
>I don't know how the shells were triggered though, maybe, when it impacts, the explosive is triggered, or a trigger on top of the shell?
The 30 mm mine shells had a chemical fuse that did not need to strike with any defined point. Its sensitivity made duds unlikely except at very shallow striking angles.
The amount of explosive carried by a 30 mm mine shell is about half of that of a hand grenade.
How many hand grenades do you think you can safely detonate inside a Lancaster fuselage? ;-)
Regards,
Henning (HoHun)