im thinking, the reason the effective radius of the shell seems
diminished might be because it was likely to be facing up, rather
than down.
Normally when a 40mm shell detonates, it would be due to it
striking a solid object head on. This means the resultant expanding
pressure is vented sideways and backwards. As the shell in question
was likely facing up, if it's used as a paperweight, then most of
the explosive pressure from the detonation would have been vented
upwards in a cone, instead of in a hemisphere towards the side and
back as it normally would. This would explain why it did not cause
as much injury as it was expected to.
Also, we have to bear in mind the probability that this was a
defective round, or that it might have been lying there long enough
that its explosive compounds have decayed and is less
effective.
