Ok, I went to the range and conducted a few experiments on a shotgun barrel and a Turkish Mauser barrel. I gained a lot of info, some I expected, some I did not. All shots from an AR 15 (223), some at 25yds and some at 10yds, amour piercing and v-max.
First, bullets react strange when they go from metal to an air gap and then impact another piece of metal.
Second, Turkish Mausers have a sleeved barrel that made for some odd terminal ballistics.
At 25 yards, both 65gr penetrators, the glancing shots look almost identical in both the shotgun and rifle barrels.
Here is a better view of the rifle glaced with penetrators at 25, I began to suspect something was up with this barrel.
Here is a direct hit at 10yds with a steel core penetrator on the shot gun barrel. It went in, disintegrated and blew out at 90 degrees from entry.
From the side, you can see the spray on the inside.
A direct hit at 10 yds on the rifle barrel with a penetrator, it peeled the outer shell off the barrel and entered bore but only made a very minor dent in the other side, lots of lead spray.
This is a direct hit at 10yds with a Hornady V-max on the shotgun barrel. It went in and denigrated, only denting the other side.
A better shot of the 10yd penetrator hit, defiantly a sleeved barrel.
Here is a direct hit with a v-max at 10yds, it went through the first layer, dented the second and peeled out of the sleeve.
I accidentally shot a v-max into the fin left by the penetrator and it peeled even more.
This experiment solidified, for me, that the hole was caused by a high velocity, small bore round. Even in very thin metal, these rounds will evaporate before leaving the other side. The armor piecing rounds will penetrate 1/4" solid plate but won't penetrate two 1/8" pieces of steel. It is entirely possible for an expanding bullet to penetrate the 1/8" of steel on one side of a barrel and disintegrate without hardly scratching the other side.