Remove the lock with an explosive bolt that triggers upon detection of intense heat and light in the internals - the hatch flies open under the immense pressure and the gases vent into the open atmosphere with minimal pressure buildup.
You could use a pressure sensor to disable the latch if it detects a massive rapid pressure build up, but the differences between a propellent burn and a detonation from super heated fuel or explosive and the hatch wont make any difference.
Load a double charge of propellent in a rifle cartridge and you might suffer pressure problems, but ironically load a quarter charge and watch the rifle blow up.
I experienced something similar when I was young and playing with fire works.
I found a skyrocket that had its fuse missing so I put it aside until I had used up all the fire works in that bag and I noticed the missing fuse in the bottom of the bag along with a large amount of black powder which I assumed had fallen out of the sky rocket with the missing fuse.
I poked the fuse back in to the sky rocket and placed it in a bottle and lit the fuse and stepped backwards. I was out in the middle of an empty field at this time and in total darkness and was waiting for a rather feeble flash of sparks, a rocket that wont go very high at all and then a colourful boom.
Instead it sounded like a stick of dynamite went off and the beer bottle I put the rocket it had its neck shattered... no light or sparks... just a boom.
A sky rocket has black powder that is packed tight and is designed in a way that the hole for the fuse is big enough to let the amount of gas the cross section of black powder release without building up to too high a pressure and exploding.
In effect it turns the black powder column into a fuse where the gas released propels the rocket upwards but the black powder only burns at a fixed rate down the rocket tube rather than all at once.
When the fuse fell out some black powder escaped and the pressed in powder collapsed over time probably creating a pocket of air at the top of the black powder column. The black powder didn't all come out because it is not sand in an hourglass, but when I replaced the fuse and lit that fuse I introduced fire into the tube.
Normally the powder is compressed so as the flame burns upwards there is no give in the powder because it is packed tight, but take out some powder and the flame can push upwards and ignite more powder at once which greatly increases the pressure... hense the boom.
With rifle ammo a rifle cartridge is a long column of powder and because of the width of the case it burns from back to front as the pressure increases.
Extra powder can cause problems because of increased pressures, but not enough powder means the wave front of burning powder goes from one end to the other too fast and all the powder is ignited at once leading to enormous pressures that can destroy the rifle chamber/bolt.
If you think of it as a surface area issue normally the surface area of propellent reacting is a circle the diameter of the cartridge case, but a reduced charge can be the length of the cartridge case at once...
But of course even just the propellent burning might create temperatures that make the fuel or HE explode too in which case blow out panels don't work.
Its propellant. It might not be readily extinguishable by depriving the source of oxygen, but you can always douse it with inert substances, like water and foams which would severely hurt its ability to efficiently combust or even self-ignite.
Really? It contains its own fuel supply and all the oxygen it needs to burn cleanly and completely and you think throwing sand on it will stop it?
It is essentially slow burning HE... do you think you could smother a HE explosion?
If nothing else its probably a bad idea to have moving parts in direct contact with the crew as they would still have to go inside the turret if they are to perform maintenance tasks on the weapon systems.
Considering all the effort to separate the crew from the ammo it really would not make sense to return the crew to the turret to fire the gun...
Perhaps progress in robots might allow a set of robot arms to be located inside the turret able to manually load the gun and fix minor problems and perform repairs...