Why are Armor Piercing Rounds Pointy?

[gibberingmouther]

A material's tensile strength only takes into consideration its thickness.

But armor piercing rounds are usually pointy at the tip, in order to focus the kinetic energy into a smaller area. Some bullets are a little pointy so they will be more aerodynamic, but I'm pretty sure the reason armor piercing rounds are pointy is different.

My guess is that less of the armor around the bullet will deform, thus less resisting force from the armor is applied to reduce the round's kinetic energy. Kevlar works because a lot of the armor fabric is pulled on by the bullet, if I understand correctly.

Am I right about this being the reason armor piercing rounds are pointy, or is there something else at work here?

 

[mfb]

My guess is that less of the armor around the bullet will deform, thus less resisting force from the armor is applied to reduce the round's kinetic energy.

Right.
Armor cannot change the overall momentum of an impact, its purpose is to distribute the impact over a larger area. If that impact happens over a small area and correspondingly with a higher force per area it is more difficult for the armor to do that.

 

[Rive]

A material's tensile strength only takes into consideration its thickness.

Piercing is more complex topic than just purely tensile strength. See layered armors, sometimes with air (!) gaps ('spaced armour').

My guess is that less of the armor around the bullet will deform, thus less resisting force from the armor is applied to reduce the round's kinetic energy.

The hole size is expected to be comparable to the diameter of the shell, so all that area will deform anyway, regardless of the 'pointyness'. As far as I know the reason is more about the fragmentation/shattering of the shell, and also pointy shells are less frequent to be deflected from sloped armors.
At the direction of 'deformation' what you will find are the modern 'APDS' arrow type shells.

Maybe you can also look up the 'ballistic cap'.

 

[Vanadium 50]

Why are Armor Piercing Rounds Pointy?

Why are nails pointy?

 

[Dr. Courtney]

Why are nails pointy?

This is the gist of it, but there are some important details. It's not the point at the front of the bullet that matters so much, as the point of the hardened armor piercing core (usually steel, occasionally tungsten carbide). US Military body armor and the highest level hard law enforcement armor are tested with the M2 AP round on the far right in the above photo. When the bullet impacts armor, the copper and lead parts of the bullet in front of the steel core are quickly flattened and the contact between the pointy core leads the armor penetration.

Layered armors have a hard outer layer (usually ceramic) designed to shatter or flatten or break the pointy tip off of the core. Of interest to armor and bullet designers and testers are the "close calls" in which the bullet is barely stopped or barely penetrates. In cases where the bullet barely penetrates, it is usually the rear of the core (with the point broken off) that makes it through. Likewise, this is what makes it deepest into armor layers when it is stopped. But other materials (hardened steel or brick or cinder block) will also often break off the pointy tip early in the penetration. So it may be debatable how important the pointy tip really is. Notice the hirtenberger design does not have one.

We conducted tests a few years back for one defense-related company where we ground the points off the AP M2 bullet cores (far right) before shooting them into various armor designs. They still penetrated really well. Consequently, I suspect that the hardness and mass and velocity of the steel core plays a bigger role than the frontal area of the point.

Newer AP bullet designs (the M855 A1, see picture below, and the M80 A1) keep the pointy penetrator, but have moved it to the very front. But in real life, the goal is not only to maximize armor penetration, but also to reduce cost, reduce weight, and increase effectiveness in living targets.

 

[Klystron]

Am I right about this being the reason armor piercing rounds are pointy, or is there something else at work here?

Note that until ~1850 most bullets and shells were round. Rifling barrels and elongating projectiles progressed to improve flight stability by spinning around the long axis. The head on the bullet and the tail (see "boat-tailed" bullets) designs are significant factors to aerodynamic stability as well. Some quite effective ammunition has round or flat faces https://en.wikipedia.org/wiki/Elmer_Keith#Keith-style_bullets.

 

[chasrob]

Weren't elephant gun's bullet rounded (not pointed) so it wouldn't glance off the elephant's thick skull instead of penetrating?

 

[Dr. Courtney]

Weren't elephant gun's bullet rounded (not pointed) so it wouldn't glance off the elephant's thick skull instead of penetrating?

Good point. The armor penetration problem is usually viewed as wanting to be optimized for normal or near-normal angle of impact.

The elephant skull penetration problem tends to require positive penetration at an oblique angle. (This skull is very irregularly shaped).

A bullet continuing along a (nearly) straight path after striking the first hard surface at an oblique angle is aided by a flat or rounded frontal profile, rather than a pointed frontal profile. We've tested this idea experimentally, and other factors being equal (bullet weight and velocity), flat and rounded frontal profiles tend to be deflected less when impacting the first hard surface.

But against harder surfaces (such as human designed armor) at near normal incidence, a pointier front is advantageous to maximize penetration potential. (Again, this assumes other factors are equal. Hardness, mass, and velocity offer more in armor penetrating potential than the frontal profile.)

 

[essenmein]

First reason these things are pointy I imagine is because it has to travel through air at very high velocity without loosing too much kinetic energy, then a bullet basically works by having such a large instantaneous impact pressure that the material its hitting yields or shears, if a round has a given mass and speed, the only way to increase the impact pressure is to make it impact area less, this means it gets long and skinny (eg sabot rounds).

 

[Nik_2213]

IIRC, it depends how you want to pierce the armour. You must have a tapered cap for aerodynamics, but bigger rounds may have a 'self forming' whatsit to burn through, or an explosive to shape and blast through.

As ever, it is the old, old battle between 'sword' and 'shield', with the usual trade-offs for weight and convenience...

 

[BWV]

This is what a modern anti-tank round looks like
https://en.wikipedia.org/wiki/Armour-piercing_fin-stabilized_discarding_sabot