20mm Oerlikon jammed and damaged

What exactly happened to this 1942 round?

The cartridge posted caused an out of battery failure as it looks as though it jammed as it was cleared and failed to eject.
The Oerlikon gun fired the primer as the cartridge was fed from the drum magazine into the chamber. This timing of having the primer firing as the round was being loaded was to help off-set the recoiling forces as the gun fired and was critical for safe operation. Any delay of the round loading would cause it to have a catastrophic case failure. The case in the photo seems to show just that. The next cartridge was being loaded as the casing pictured was trying to be ejected jammed in the gun. The round going into the chamber was ignited and the case ruptured and blew the gasses downward ( as designed)The high pressure gas tore into the jammed casing and folded the brass side wall inward and produced the “wall” of brass you see. The finger looking piece is an exact silhouette of the bolt’s ejector as it would have deflected the high pressure gas as it passed.
A very interesting failure of the gun.
The Oerlikon gun was a well designed weapon and used all over the world in WWII. The timing issue/failure was a problem with these guns firing as the case was being loaded. It was the guns weak point.

That case has the appearance of a misfed round jamming, not a fired case. The neck of the case has not been blown outwards under pressure but the projectile has been bent upwards and thus came free. Please can you confirm whether the primer was struck, or, better still, show an image of the base and headstamp?

This type of misfeed can occur if there is insufficient magazine spring tension. If mag spring tension is low (it should be wound up by a crank after filling with live rounds) then the round being rammed bounces on ahead of the bolt face in an uncontrolled manner. The extractor of the Oerlikon is a solid machined lip at the bottom of the breech face and the cartridge should slide down the face as it moves into the chamber and engage behind the extractor. If the cartridge rim gets ahead of the extractor then it will jam and cannot be fired. The firing pin/striker of the Oerlikon does not protrude from the breech face until the cartridge is well inside the chamber.

In this case the round seems to have got part way into the chamber before the breech block muscled it aside. The odd shaped hole in the side was caused by the feed face of the breech cutting into it as it was mangled. The “tongue” was left uncut by the groove for the ejector. The brass cut is now folded into the forward part of the case. The denting on the opposite side of the case was caused by the mouth of the chamber.

It is normal practice to leave filled magazines with low spring tension, enough to prevent rounds getting jumbled, to prolong spring life. But they must be “wound up” before shooting. If action is imminent then, of course, the mags are tensioned.


Please reread my post. It is an empty case that was jammed and cleared but did not eject that was under the live round that failed.

This is the headstamp. I looked inside at the base, I can see 2 flash holes and a very clean base. This round was a part of a large box of military ammo and various machine gun belts. I have absolutely no history of it. I do not understand the sentence “The Oerlikon gun fired the primer as the cartridge was fed from the drum magazine into the chamber.” I never saw such a firing device, in all my guns, like Mosin-Nagant or SKS, a primer hit in an un-chambered round would equal disaster. Please, explain this sentence in detail.

SKSVLAD- You may find Tony Williams’ excellent book “Rapid Fire” to be worthwhile. It covers nearly all the modern rapid fire guns, their technology and fire control and mountings. Really good stuff.

And, yes, the Oerlikon does fire on the final phase of loading, which is very counterintuitive, but when it works as designed is pretty slick. However, when something goes wrong… you see the result.


Maybe this wiki description explains it better:

[color=#0000BF]The Oerlikon cannon and its derivatives feature blowback operation: The bolt is not locked to the breech of the gun on the moment of fire, but moves freely to the rear while the propellant gases propel the projectile forward. Advanced primer ignition (API) is used to make sure that the projectile has left the muzzle and the gas pressure in the barrel is down to a safe level before the breech opens, the firing pin strikes the primer while the bolt is still traveling forward so that the gas pressure first has to overcome the forward momentum of the bolt before it can push it to the rear. To give the heavy bolt sufficient forward speed, a large spring is required, which is wrapped around the barrel of the gun. Also, the chamber is longer than needed to contain the case, so that the bolt and case must travel a small distance to the rear before the case extends beyond the face of the chamber. Nevertheless a fairly heavy bolt must be used, which limits the rate of fire.[3][/color]

The same type of operation is used in other firearms, most notably the M1A1 Thompson and M3 “Grease Gun”, which have no breech locking provision or seperate firing pin.


An explanation taken from the manual of 20 mm cannons Mark 2 & 4 (1943):