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Comment by kibwen | Hacker News Reader

Neat, though of course this raises the question: what would it take to engineer a foam-piercing bullet?

Two projectiles. First one explodes on impact and compromises the target. Second one penetrates the target.

This is exactly how a Soviet style RPG works. Ironically, it is also why the "bird cage armor" works so well against the RPG. Here is a picture of a Stryker combat vehicle from when I was in Mosul Iraq circa 2004ish:

http://www.digitalprognosis.com/album/images/iraqpics/the-st...

The slats are smaller than the height of the outer explosive and prematurely detonate it. As a result, the penetrator simply bounces off the armor harmlessly.

I thought the slats were to provide stand off distance for explosively formed projectiles.

If the explosively formed jet forms more than a few inches from the armor, it's way less effective.

The Vampir uses a tandem charge -- Two successive charges. The point of this is to counter reactive armor, which is designed to counter explosively formed projectiles.

The "cage" slats can be effective against both.

That's backwards. Explosively formed jets need space to form, so work better if they detonate a little distance from the target.

No, you're right. Explosively formed projectiles work best when formed a few inches from the armor. However, they have a sweet spot and if the distance is too far, they don't work. I figured that's what the cages were for.

Nope, we had them in Iraq before EFPs were really a thing (Thanks Iranain Quds Force for that). They were specifically for RPGs, which was one of the few things that could take out a Stryker without it. They even tried those Soviet parachute anti-tank grenades, but the Stryker is much faster than a tank and just moved away in time. That being said, the I'm sure this slat (birdcage as we called it) armor would help with that as well.

I thought an RPG _was_ an EFP? The round is just a shaped charge with a copper liner that penetrates the armour. So the slatted armour stops both RPGs and EFP-based IEDs.

The usual RPG with a HEAT shaped charge doesn't destroy by forging a penetrator that then potentially travels some distance to the target, it produces an intense, narrow jet that'll cut through a lot of material.

Not really. EFPs (in Iraq) were more or less IEDv2 in that they were simply much more lethal than the original IEDs:

http://www.cbsnews.com/news/us-sees-new-weapon-in-iraq-irani...

I though slat armour worked because RPGs only had a detonator in the nose, so unless you were very unlucky and the tip of the RPG hit a slat, the RPG would just jam itself between two of the slats.

Then they make the new guy get them out with a pry bar.

I saw that a few times actually, but only when the idiot firing the thing didn't pull the safety pin out of the warhead. The RPG is surprisingly safe when you neglect to remove the safety and will happily fire, but not explode in that circumstance. It was around the height of the Fallujah offensive we saw that happen.

...which is already defeated by reactive armor, which you'd presumably layer underneath this ablative mesh armor layer to prevent this kind of attack.

So, three projectiles in the same spot?

Better start with four so you're ready for the next shielding upgrade.

How about 180?

https://www.youtube.com/watch?v=xe-QP8s1ZJc

I've seen that before and I am so disappointed in the slow motion part. I wish I could see those bullets going one behind the other in a perfect 8 pattern.

I'm not an expert, but after some Googling, I think the bullet they're using there consists of a steel core inside a lead envelope inside a copper or copper-nickel jacket. So, at a guess, you'd want to replace the steel with something much harder and/or denser. Tungsten, maybe? I dunno. Depleted uranium would probably do it; I've never heard of it being used in anti-personnel rounds, but in cannon sizes it pokes holes in tanks quite nicely.

There are several rounds the military uses for small arms. 50 BMG and 5.56 NATO both have tungsten versions, though they are expensive, and hard for non-military to buy (when found, $50/rnd). Even the military does not normally issue them due to the cost, which is ~2-10x of a standard lead+steel round.

Indeed. The AP M2 round is the standard for National Institute of Justice (NIJ) Level IV rifle plate armor, but to the best I've been able to determine it's not a "serious" AP round, rather, it has a pill of relatively soft steel inside the normal lead and was developed in the run up to our involvement in WWII as much for the lower cost as the increased penetration. The latter of which was appreciated towards the end of WWII when I've read it became ubiquitous, and for example in city fighting one or more 20 round BAR magazines of this were used to knock holes in masonry walls between buildings.

It was probably made the NIJ standard because it's the only somewhat common AP ammo a US cop is likely to go against on the street, as noted by extrapickles the serious AP ammo is a lot more expensive, and I'll add not really legal any more since there are some pistols that'll fire it.

Notably, ESAPI military equivalent plates also aren't certified to stop anything more serious than AP M2, and I assume again for the same reason, there is or was a lot of surplus AP M2 ammo out there at one time. In the US civilian marketplace it's pretty much dried up as I understand it, e.g. see the few hits this GunBroker.com search finds and the prices: http://www.gunbroker.com/Ammunition/BI.aspx?Keywords=ap+m2 and I can't find any demilled bullets there or with a couple of minutes of Google time, whereas they were commonly available not too many years ago, certainly within the last decade.

A bullet made out of metal foam?

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