Air space blows up modern rifles and cannon - howitzers, for example - as well. Some problem in mountainous Korea, high angle fire w relatively little propellent.
Here are a couple of older quotes for yor digestion:
Air Gaps
An air gap between powder and ball can spell trouble. This is widely believed and very wisely accepted as fact. Ezekiel Baker’s remarks on this subject still bear repeating: “Every rifleman should mark his rammer at the muzzle end of the barrel, when loaded, which will shew him when the ball is close down on the powder. After firing a few rounds, the filth from the powder will clog at the bottom of the barrel, and prevent the ball from going close on the powder; in this case, a little pressing with the rammer will be required to get the ball into its right place. More accidents happen from a neglect of this precaution than can be imagined: if the ball is not rammed close on the powder, the intervening air will frequently cause the barrel to burst; not, I confess, that there is so much danger with rifle barrels as with fowling pieces, the former being made much stronger . . . “
Ned H. Roberts also cautions “. . . if the rifle be fired with the ball partly down, you will soil the barrel by enlarging the bore, or making a “ring” in it, at the point of the obstruction.”
The best modern published work is that of Carl Wood, who showed that a 1/4” air gap could double the Breech pressure in a .40 caliber. Seating a .40 cal. ball just 1/4” ahead of 50 gr. FFFg resulted in 43,150 psi at the breech. Comparable to smokeless pressures in a .44 magnum.
This pressure was measured at the breech. However, when a barrel is ringed or burst by an air gap, that damage is said to occur at or just behind the ball. This means that the highest gas pressure is just at or behind the ball, and not at the breech. An air gap between powder and ball is clearly dangerous. And this pressure will be higher than whatever is measured at the breech.
Frank C. Barnes9 covers the air gap problem briefly. When discussing loads for the .45-70, he warns “. . . don’t use cardboard wads seated on top of the powder with a large air space between the wad and the base of the bullet. This practice has been known to bulge or ring the barrel in the forward area of the chamber.”
Why does this happen? One description given by Weldon H. Clark, Jr.,10 whose Master’s thesis was on wave mechanics, goes as follows: “If there is a column of air between powder and ball, the following series of events are very likely. The powder charge is ignited. The burning powder causes a pressure wave to travel down the barrel at a very high speed. When the pressure wave meets the ball, the ball acts as a solid wall (in wave mechanics, materials which are much denser than the gas are seen as a solid wall). At this instant, a reflected wave travels in the opposite direction back toward the powder charge. At the rear of the ball, the pressure becomes the sum of both waves. At a solid ball, the reflected wave is of equal strength as the incident wave, therefore, the pressure right behind the ball is double the pressure generated by the powder charge.”
A similar phenomenon occurs every once in a while during explosive cladding of nickel alloy plate onto carbon steel plate. Two plates are laid down on hard, solid ground, one on top of the othr. A mixture of ammonium nitrate and fuel oil (high explosive) is poured onto the top plate. A wooden framework around the plates keeps the explosive from spilling over the edges. The explosive is detonated from one end. As the shock wave travels along, it literally welds the two plates together by intense pressure. Once in a great while a shock wave travels to the end and REFLECTS OFF THAT WOODEN FRAME. It then travels back toward its start and meets a second shock wave. The pressure at that point is twice normal and that is indeed the end of those plates.
Explosives do not always behave the way they are supposed to.
A problem with trying to measure the effect of an air gap is that the area where the pressure is doubled is “right behind the ball,” and not necessarily at the breech where pressures are measured. Further, there is no guarantee that this pressure doubling will occur each and every time an air gap is present.
When such pressure doubling occurs, the result may be a ringed bore or burst barrel.
I have seen more than one such burst barrel and heard of the permanent effects on the shooter. The
first was a pile of steel pieces, many smeared with a reddish-brown substance. Still gives me bad feelings to recall.
first was a pile of pieces, many smeared with a reddish-brown substance. Still gives me bad feelings to recall.
