Educate me - API rounds


I’ve had this question for quite some time and it’s been bugging me.
We`ve all seen the typical configuration of API rounds, with the steel core having a “cap” of incendiary compound.
My questions are:

  1. Is it always placed in the tip? (not taking in account fuzed incendiary shells)
  2. If yes, does it have to do with the way the compound is ignited, i.e. by the extreme compression provided by the penetrator? Or is it ignited in some other way?


Razvan, when we exclude Wp fillers there is two designs - or a combination thereof.

One is the initial design where the incendiary was placed infront of the core.
The other is where the incendiary is placed behind the core but also may have additional incendiary infront of the core too.
Ignition depends on several factors in my view. While “front incendiaries” are more likely to be ignited by the impacting core and compression on the target material + deformation of the jacket - thus creating heat, the “rear incendiary” is more likely ignited by the deforming jacket and by the core path inside the target material which got heated up by the driven out material (the denser, the more heat).

The most prominent “rear” API types are the Soviet designed 7.62x39 “BZ” and the 7.62x54R “B-32M”. Both have the incendiary behind the core and the B-32M has it also in front of the core.

7.62x39 "BZ"

7.62x54R “B-32M” (image source: internet)

Thank you for the explanation, Alex!

What i`m more interested in are the “front” ones, and more specifically, how much importance does the penetrator have in the ignition of the compound (actually, the compression provided by it).

There used to be, especially in large coastal artillery pre-1900 some armor piercing shells that were filled with black powder (or in some cases, black powder mixed with aluminium powder) that would ignite themselves on impact without a fuze. The explanation provided in the era is that the intergranular friction upon impact generates sufficient heat to auto-ignite the filler. That seems a bit far fetched but at the same time it could be possible, but maybe just in very large calibers (>150mm)… I was wondering if this principle is true, and whether or not it could work with incendiary compounds found in modern era ammunition (12.7mm and bigger). With or without the penetrator providing compression.

I think it also depends on what the incendiary compound is used… Too many variables.

Razvan, There are also incendiary and spotter rounds which use solid incendiary compositions and ignite upon impact without a core.
Please keep in mind that forces and pressures on impacting projectiles are extremely high and that a projectile jacket deforming heavily or rupturing creates immense heat in a fraction of a second.
This then is sufficient to ignite the incendiary.
A core does not matter at all. This is also why “rear incendiaries” are working too.

The old artillery shells with BP filler and no fuze were the children of their time.
I am not sure if it was only the impact shock and friction or also the heat generated by penetrating the armor.
Here I am not informed enough. Maybe somebody is willing to look up old text books to find the explanation used those days.

But once you are on it you may think of German 20mm API of WW2 times. And there the 20x138B API-T and 20x82 API. Both had a WP filler and a base plug, no separate ignition or bursting charge.
The described principle was that shock waves and heat would cause the base plug to pop out and the WP (in late versions all encapsulated in on little bullet shaped steel or aluminum capsules) to burst out of it’s small capsules. Somehow it must have worked as these were made by the millions for about 10 years.

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Thanks for the great explanation, I totally get it now!

Actually, you are correct with this: in the era, both explanations have been vehiculated in a manual regarding the Romanian 150 and 210mm guns model 1887 that used such shells.

Now you have me intrigued - how does one section an incendiary cartridge and live to show it to anyone?

Discussing the specifics of sectioning is not allowed on this forum but suffice it to say that the typical techniques used do not generate sufficient heat or friction to ignite the incendiary mix. WP fillers are a whole different story though.

Black powder is insensitive to friction. If you add Aluminum powder you now have Flash powder which is extremely sensitive to both friction and heat. Although flash powder has been around since the 1870’s, it really didn’t come into play until the 1920’s for other than photo flash.

As far as I know, photo flash used magnesium powder.

Razvan, photo flash powders exist in a variety of compositions (usually with added chlorate and other somponents), usually they are not magnesium powder only.
This is why these powders are so sensitive.