What a very strange question. You're describing explosions of
devices. The cartridge, the M-80. And you're describing associated
devices being damaged. The gun doesn't explode, it is damaged by
the 'exploding' cartridge. The mailbox doesn't explode.
The 'relief channel' is designed into the device, such as the gun, But
let mud get into the barrel, and watch the force get vented otherwise.
The door of the mailbox doesn't have to open. The box may or may not
be deformed/damaged by the M-80, with or without the door opening.
In fact, the box could be damaged by the localized explosion, even with
the door open.
I'll not tell you where I got my training with explosives, but I can tell
you
that they've been in business for a long time, and that their training is
frequently called upon to change someone's lifestyle.
I will again endorse the choice of using stump rotting chemicals, and
ignoring you.

SteveB wrote in message
news:rastb.14271$Q64.1419@fed1read03...
As in when a cartridge in a gun is supposed to fire, and the "cork" (lead)
pop out through the barrel, but doesn't and causes the gun to "explode"?

As when a blackpowder M-80 is put into a mailbox, and the door doesn't
"pop"
open to relieve the pressure, but the mailbox is destroyed?

Things explode all the time although they have a relief channel.

Where did you go to explosives school? I think you should ask for a
refund.
What state do you live in? (other than the state of confusion) I want to
avoid that state in my travels.

Steve