Try sitting inside an M48 tank when the main gun is fired. Inside a hollow steel turret, that sound inside doesn't have anywhere to go, and just sort of bounces off the interior walls until it finally dissapates. But you WILL know when a main gun round is lighted off....
-archy-/-
ON THE WAAAAY
Target, cease fire.
Back in the 'dark ages' (when I was in high school ;>), I had a history instructor who had been an Army officer, serving with an armored division during a 'Berlin crisis' (he was also a Viet Nam combat vet). He told me that they had faced Soviet armor across the border - with only HE ammunition available in the American M48s. No AP, just high explosive. The Soviets presumably had issued AP to their armor units... but nevertheless backed down. Which just goes to show, those American tankers may have had inadequate ammo, but they had 'balls of steel.'
(Oh, the reason for this post: the gentleman in question was still in the reserve, and mentioned that he had experienced "hearing loss" from the 90mm gun in his M48... ;>)
(A good man - and one reason I took an interest in history...;>)