Shotguns are used by a lot of people because they are easy to operate and guaranteed to be effective. I have read about cops putting 6. 38's into a criminal on PCP and it didn't even slow them down. Needless to say a 12 gauge would be much more likely to stop someone on drugs like that.
I would get out of there after hearing a a shot as well but you and I are not criminals. One of them could have just as easily pulled out a gun and killed the junkyard owner after hearing the shot.
A different weapon may have been less deadly but you never know. He could have used a AK-47 or. AR-15 and the outcome probably would have been the same. Likewise a. 357 or. 45 isn't exactly non lethal.
So, pretty safe bet to say junkyard dude must've had buckshot loaded?
We shot a few 'roos on a farm up north with a .243, pretty devastating, definitely not a pretty sight on the exit side.
Even if all you had are these massive calibers (note: PJ, dunno why in your scenario he's always gotta be equipped with something that could take a friggin elephant* down), even if you needed to be sure you were gonna kill the guy when you fired, *why* wouldn't you fire a warning shot first, knowing should you actually fire you're likely to kill? You'll still have the drop on them should you then see that they're armed, but if they're not and they turn out to be 16yo's and they just start running, then you'll have left them their lives/limbs.
*obvious exaggeration lewl
I have seen .45's and .357's rip things apart from 50 yards. I guess it does depend on the gun firing the round as well. My Grandfathers 6" barrel .357 is a lot more deadly than my buddies 3" barrel. Even though they are common calibers, they leave large holes. Just because there are higher caliber weapons doesn't mean those aren't a high caliber.. Also, I have seen an m1911 do crazy amounts of damage to a television at 25 yards.. LOL good times..
I am saying with a HUGE amount of fire arm experience and ownership.