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Okay, let me show my ignorance of guns and ammo.

I was under the assumption that older, black-powder guns had a slow rate of powder burn, so slow in fact that some residue of the charge was still burning as the round left the barrell.

Now, newer guns use much faster powder.

I would be surprised if powder was still burning, TO THE POINT OF INCREASING MUZZLE PRESSURE, when the round left the end of the gun.

I'va always assumed that maximum muzzle pressure behind he round was acheived early in the bullet's travel down the barrel. Is that wrong?

Ideally, where in the bullet's travel down the barrell is maximum muzzle pressure achieved?

I don't understand how the relatively short distance that the extra-insertion of a round into the casing experienced could account for THAT much overpressure. Once the powder starts burning and casing pressure starts rising, the bullet begins to leave the casing. As the distance increases, more powder has to burn to keep increasing pressure.

In my admittedly shallow bit of knowledge on guns and ammo, I would have thought that maximum pressure would be achieved several inches down the barrell, not at the point where the powder first began to burn.

It almost sound like the bullet is getting stuck in the casing and will not expel properly from the casing.

All that flash you see when you fire a short barrel pistol is unburned powder. Continued powder burn is also one reason rifles will have a higher muzzle velocity shooting a pistol round. Black powder is classified as an explosive because it burns very rapidly.

The highest pressure is typically right at the point before the bullet starts to move. Handgun powders burn pretty fast, so that pressure peak is reached rapidly. Rifles uses a slower burning powder, and tend to have a longer time of high pressure. But, since a handgun has a small barrel, you need all that pressure fast to get the bullet up to speed before it leaves.

The .40 is a "high pressure" handgun round, that is a lot higher than say a .45 ACP, so there is less margin of error. Compressing the bullet in a loading with fast powder makes the pressure go up a lot, sometimes enough that the case will rupture before the bullet has moved far enough to allow the pressure to go down. There just isn't enough room for the burning gas, so you get a Kaboom.

Glocks, it seems, are known for this, rightly or wrongly, but it can happen in any handgun or rifle. I saw a similar (but less dramatic) thing happen with a 1911 once.