AA Shells Detonation

[alamotex]

I was looking at some data re WW II bomb damage to a coastal UK town that was not the target for heavy Luftwaffe air raids. I wondered how many of the bomb damaged buildings were actually struck by AA shells that failed to detonate in the air and exploded on impact when they returned to earth ?

Just how reliable were the WW II timed fuzes fitted to heavy anti aircraft shells and if the shell failed to explode in the air, would it explode on impact with the ground ? Ground damage from exploding 3.7", 4.5" or 5.25" shells were unlikely to be reported ,as such, in the local press at that time.