WWII weapons question: Why not anti-tank rifle grenades?

[Magniac]

A very simple thought I had when watching Mythbusters. Tory is able to fire a harpoon/rope-attachment one hundred feet from a modified Martini rifle, hit a target one foot in diameter, all on the energy produced by a .38 Special blank cartridge.

Which begs several questions. If you're the British military in WWII, why would you produce something as cumbersome as a spring-activated PIAT to fire a shaped charge when you can just stick an ordinary longarm on a bipod, or even in the PIAT itself, for the same effect? A simple scaled-up version of what Tory had. That is very obvious (though admittedly I think rocket-grenade tube technology, ala bazookas or panzerschrecks, is still the most advanced man-portable ordnance possible for the Second World War).

But that's nothing, as there is a much broader question: why can't anyone use shaped charged rifle-grenades as a simple panzerfaust system?

So, when can you equip every infantry platoon with this panzerfaust equivalent system, in what theatre, and to what effect?

Perhaps such a weapon couldn't be a standard rifle, but would need a redesigned barrel, like the harpoon weapon.

 

[Larrikin]

The British did have anti-tank rifle grenades during WWII, but those lots the gun-armour race fairly early.

Defeating armour in WWII can be done 3 ways -
1. Kinetically, you hit your target hard enough you can punch through
2. HE over match, where the explosive power is enough to just plain break the armour
3. Shaped charge, where you focus the explosive power narrowly.

The penetration of a shaped charge is defined by the diameter an depth of the cone, the greater they are the stronger and longer the plasma jet generated. With a rifle grenade you are limited in both diameter and depth, and so it becomes almost impossible to produce a jet that has the penetration to go through much more than about 40-45mm of amour, and that is with a perfect perpendicular hit.

Rifle grenades are also not all that accurate, and when you are using either a blank or a bullet trap set up to fire them they kick like a mule, and thus become even less accurate.

Then we get to the Projector, Infantry, Anti Tank, aka the PIAT. It actually had some significant advantages over the bazooka and panzershreck. It was less cumbersome than either, and thus easier to lug around. It didn't have a back blast, so you didn't have to worry about one of your mates being directly behind you, and you could use it in a confined space, like a room or a gunpit with head cover. And it didn't leave a nice flaming trail through the air at night, which meant they were ideal for night operations, as the were also relatively quiet.

 

[Cockroach]

Given the general idea of a rifle launched HEAT grenade is feasible (post-war the Energa rifle grenade proved popular in Europe, and it was moderately capable -able to penetrate up to 200mm of armour in early versions and up to 275mm in later versions) I'd have to guess there are a few factors:

1. The understanding and development of shaped charges improved a hell of a lot over the course of the war and the immediate post war years.
2. The rather dramatic increase in armour thickness over the course of WW2... Looking at the British cruiser series for instance you got from 14mm max armour thickness in the pre-war A-13 to 76mm on the Cromwell (upgraded to 100mm in some cases) to 150mm in the early versions of the Centurion (with the added bonus of sloped armour). Basically, you design a grenade in 1940 that should be capable of killing any tank in service, by the time it is in production in late 1941 it's now only of use over the side and rear arcs of most new armour. By 1943 it is totally outclassed .
3. Limited range: comparing post-war developments of the Bazooka to the Energa you find the Bazooka has about three times the effective range and twice the maximum range. Compared to the late-war M20 75mm Recoiless Rifle grenade performance is even more pathetic.
4. During WW2 most armies has extensive AT support in the form of towed AT-guns.

 

[Magniac]

Rifle grenades are also not all that accurate, and when you are using either a blank or a bullet trap set up to fire them they kick like a mule, and thus become even less accurate.

Generally this is right, which is why I included that brief speculation in my OP about a PIAT with a rifle-grenade propulsion system (a rifle "even in the PIAT itself") being an improvement on that particular weapon, and why I mentioned the panzerfaust.

Neither the PIAT nor the cheap German weapon were hugely accurate or longrange weapons, though the panzerfaust had pretty manageable recoil I suppose.

Given the general idea of a rifle launched HEAT grenade is feasible (post-war the Energa rifle grenade proved popular in Europe, and it was moderately capable -able to penetrate up to 200mm of armour in early versions and up to 275mm in later versions) I'd have to guess there are a few factors

That's the weapon system in the above image. I have to admit I just went online looking for any old picture of a shaped-charge rifle combo, I didn't look for specifications.

I think the point about the rapid advance in AV armour thickness during WWII is the most important consideration. Anti-tank weapons are only as effective as the strength of the explosive they deliver.

Now, I do think a rifle grenade system that is comparable to the simplest model of the panzerfaust has the potential to be, what, a pointer of things to come? A cheap and nasty success?

On second thought the panzerfaust analogy falls down a bit, as I think it was only ever an important issue for Western Allied tank forces who suffered from low moral during during '44/'45.

Otherwise I don't know if an Energa rifle grenade issued in the hundreds of thousands to Red Army infantry before Barbarossa is plausible, or if it has any effect on the German advance.