"2 Pounder HE Rounds, Little John Adapter, Ammo Effectiveness" Topic

Yes, it's ballhooks. HE was produced and used. A friend even has a 2pdr HE round – marked as manufactured in 1942 iirc. The same friend also owns the world's only surviving 2pdr Littlejohn HE round – it has a crushable mesh outer sleeve, to theoretically allow it to pass through the adaptor. Only three prototypes were produced and the first two blew up in the barrel… :o)

As you say, regiments equipped with Littlejohn actually started removing Littlejohn as 2pdr HE became more available. The AP advantage given by Littlejohn was overkill for light AFVs, but still wasn't enough for tanks, so it was seen as pointless – especially when HE appeared. Sorry I don't remember the source, but I read of one armoured car regiment having a 50/50 split of Littlejohn and non-Littlejohn cars. 2nd HCR reported knocking out an 88 across the Waal at Nijmegen using 2pdr HE.

Yes, it's ballhooks. HE was produced and used. A friend even has a 2pdr HE round – marked as manufactured in 1942 iirc.

Yep. 2pdr HE was produced all along.

But … it was originally issued only to the artillery units equipped with the 2pdr. Not to the tanks. This was in keeping with doctrine, which was that tanks were to close with the enemy, while artillery was expected to stand-off and shell the enemy in support. Even close-support tanks, which replaced the 2pdr with 3-inch howitzers, were originally issued with smoke to assist in closing with the enemy, rather than HE to assist in KILLING the enemy.

It took a lot of real-world experience to get that doctrine out of the minds of the logistics guys.

The AP advantage given by Littlejohn was overkill for light AFVs …

Actually, in fact (though not in perception), the Littlejohn was not overkill. Rather, it was under-kill.

A squeeze bore does nothing to accelerate a round.

Rather, it decelerates it.

What accelerates the round is having a light round relative to the bore size. Big base area to push against, light weight for minimal inertia, and you get a real popper out of the muzzle.

Now make a tapered bore, worse yet as an extension onto an already well-designed barrel (of appropriate length), and you require more work to overcome not only the added friction but also the crushing of the projectile's flanges.

If you fire a 2pdr APCNR round out of a non-squeeze barrel, it will come out of the muzzle FASTER than if you fire it from a squeeze barrel. Notably faster.

The one advantage of squeeze-bore was that, after it left the barrel, it was no longer a round that was light in weight relative to its bore, but rather was a round that was heavy in weight relative to its bore. Because, of course, it had the same weight but a smaller bore (it had been squeezed). So the squeeze had the effect of changing the round from a low inertia round to a high inertia round. It went from easy to accelerate, to hard to decelerate.

In this issue projectiles fired from squeeze bores behaved like later APDS rounds, retaining their momentum over range better than APCR (HVAP in US parlance). However, the process of squeezing meant they started at a lower muzzle velocity than ACPR fired from the same gun, and so the cross-over point in striking power was further out than APDS vs. APCR.

With a 2pdr, who really cared that you had marginally better performance after about 1,500 or 2,000 meters if you used the Littlejohn adaptor? What they wanted was performance at 300 to 800 meters, which they got if they removed it. But they didn't recognize that the performance was better, only that it was not worse. So the fact that they got the ability to fire HE too was what led to the dis-favor of the adaptors. The fact that the APCNR rounds seemed to still work very well (actually better!) was only a happy coincidence.

The notion that the squeeze of the bore would increase the pressure behind the projectile was a red herring. No matter how much you squeezed it, the volume of the barrel always got larger as you travelled up the bore, so your pressure always fell. So all you were doing by squeezing was making more resistance to the projectile's travel while in the barrel. All of this was not understood at the time. Much like the question of barrel length … oh, if an L48 gives better performance than an L43, then an L70 will work better than an L56, right? Nope. See any guns with barrels longer than L62 today? There is a reason. See any squeeze bores today? There is a reason for that, too. It is not that we aren't interested in higher velocities -- quite the contrary. But now we understand what actually works, and what doesn't.

Now we understand the dynamics of what happens in the barrel far better than they did in 1942.

-Mark
(aka: Mk 1)

The HE was eventually produced once the gun itself was out of the picture for the main part. By the time the HE came out, only armoured cars still used the gun. Think of the effect of the HE round as probably a little less than the current M203 round in results.

Strangely enough, a makeshift HE round was developed in the South Pacific for use by Matildas. Somehow a Bofors projectile was Heath Robinsoned into the casing and made to work.

Thanks Mark, that's absolutely fascinating. So didn't they do armour-penetration tests on these things before issuing them?

Any information on how the Germans viewed their 28mm and 42mm squeezebores?

Agreed, the HE went initially to AT Regts RA. I've never found any anecdotes or other evidence for its use prior to 1943, though it was widely used among armoured car regiments and the like in 1944.

Troopwo, that's interesting. 7th Armoured Brigade arrived in Burma in 1942 without HE rounds, even though they were equipped with 37mm guns, for which HE was readily available!

Seconded re Mark1 – could you tell us more about the L70 and why it wasn't as good as expected, I've not heard this before?

I believe that 2pdr. HE was produced and issued before the war, and withdrawn from tank units about Oct. 1939. After all Vickers was making HE shells for the Pom-Poms, so why couldn't that be used on the AT gun? It was a doctrinaire decision; nothing to do with the capabilities of the gun or industry.

I've often read of tank crews firing at German targets in the Spring of 1940 and witnessing explosions. That would tend to indicate HE, no? So, maybe the crews/units didn't turn in all their HE rounds?

As regards the Bofors rounds for Matildas, I had read that it was U.S. 37mm rounds with a white metal driving band shrunk onto them.

I believe that 2pdr. HE was produced and issued before the war, and withdrawn from tank units about Oct. 1939. After all Vickers was making HE shells for the Pom-Poms, so why couldn't that be used on the AT gun? It was a doctrinaire decision; nothing to do with the capabilities of the gun or industry.

This is also my understanding.

I too have seen/read accounts of 2pdrs firing HE in 1940 in France. It was only in the Western Dessert that the 2pdr equipped tank units suffered for the lack of an HE round. Yet the AT regiments at that time had HE for their 2pdrs. However, cooperation or conversation between the armored and artillery boys was just not the thing in those days.

…could you tell us more about the L70 and why it wasn't as good as expected …

Technically there are two issues at play -- first is acceleration vs. friction. Second is barrel warp.

How a projectile is (or isn't) accelerated was not examined in detail by armies prior to WW2. The general presumption was that chamber pressure equals pressure on the base of the projectile. If you have a high enough chamber pressure you are pushing the projectile, and hence accelerating it, more than the friction is decelerating it. So you just do some math about how much bore volume there is vs. how much gas you have released through burning the powder and voila, you have an easy answer about the pressure applied to the base of the projectile. Put in enough powder and you can make a barrel of infinite length.

The problem is that the burning powder, and hence the expanding gasses, are in the chamber at the breach. But the projectile can be a LONG way away from the chamber and still be in the barrel. It takes time for the pressure wave from continued expansion of the gases (continued burning of the powder) to reach the base of the projectile.

In highly simplified terms you have X + Y, where X equals the distance your projectile travels in the barrel where the pressure from the gases exceeds the friction, while Y equals the distance your projectile travels in the barrel where your pressure from the gases does NOT exceed the friction. You want all X, and no Y. But there is almost no path to get all X and no Y after about L60 to L65 length. On smaller bores maybe L70, on larger bores closer to L55. The problem is that the pressure wave released from the burning powder just does not travel fast enough to reach the projectile before it exits the barrel.

The Soviets tried to improve this phenomenon in their 115mm and 125mm guns by carrying the powder charge with the projectile as it travels up the barrel on their APDSFS rounds, so that the pressure wave was right there where it mattered. But this had adverse effects on accuracy, because it takes some distance for the turbulence of the burning powder to meld together into a smooth pressure wave

So today you will see big bore tank guns (like the current generation of 120mm or 125mm) don't go above about L55. Smaller guns, like the 60mm hyper-velocity guns, may go up to L65 or so. Down below 50mm you might even find some L70s.

A very interesting example of this is the French post-war 75mm gun. They studied the German 75mm KwK42 gun (the L70 in the Panther) in detail. They built their own gun to duplicate it, which is often asserted to be a copy but was actually a 100% original French design. They built the cartridge to match the Panther's performance, but built the gun as an L62 rather than an L70. The result was a better gun. More accurate, with marginally higher performance.

Now why was it more accurate? Because of the second issue, which is barrel warp. Longer barrels warp more. They weigh more, which tends to pull the muzzle down. They have more surface area exposed to the sun, which tends to heat up and expand at a rate which is inconsistent with the expansion of the lower half of the barrel, causing the barrel to bow upwards and again pushing the muzzle down. The heating and cooling after firing is never consistent over the length of a barrel, and the longer the barrel is, the more amplified any firing-induced warpage becomes.

The net result is that any barrel longer than about L50 will be prone to warp-induced inaccuracies. On a sunny day the gun will strike higher in the morning than the afternoon. It will strike lower on cold days, particularly when ambient temperatures are below freezing (which adds stiffness to the metal of the barrel). And it will fire further off target in almost random directions (sometimes compensating back up for the down-warp from sunlight) as progressively more rounds are fired in any one engagement sequence.

That discussion though focusses only on technical accuracy, which is measured by dispersion. Practical accuracy also must take into account velocity and the trajectory that results. Firing a faster round means a flatter trajectory, so that errors in range estimation have lesser impact. This is not taken into account in measuring a gun's accuracy technically, but was much praised by combat-experienced gunners up until the time of laser range-finders.

The 88mm L56 in the Tiger was noted as an exceptionally accurate gun. The L71 in the Tiger 2 was less so. In fact it probably matched the L56 during winter, or with the first shots in the early part of a summer day. But these patterns were not understood. So rather, it was viewed as less consistent. The flatter trajectory meant it was still effective in longer ranged engagements, even though its accuracy declined as more rounds were fired.

In modern guns the L44s are actually more accurate than the L55s, but this is a marginal effect because of modern muzzle-reference systems in the integrated fire control systems, which measure and compensate for barrel warpage by adjusting the point of aim in the sights.

Any of our members who took the full master gunner's course at Ft. Knox can probably add to this information. I only had the pleasure of a master gunner's orientation…

-Mark
(aka: Mk 1)

Thanks Mark, that's absolutely fascinating. So didn't they do armour-penetration tests on these things before issuing them?

They certainly did. And they found that the Littlejohn firing APCNR performed much better than the standard 2pdr firing AP.

What they didn't do was test the APCNR fired from through the Littlejohn adapter against the APCNR round fired from a standard barrel without the Littlejohn adapter. At least, I've never seen any indication that they did.

But why would they? The APCNR round was developed specifically FOR the Littlejohn adapter. Why test it without?

See, getting the answers is not the hard part. Figuring out what questions to ask is always the challenge.

-Mark
(aka: Mk 1)

FWIW the perceived improvement in the anti-tank capability of the Littlejohn 2 pdr caused the 90th Atk Battery of the 1st Anti-Tank Regiment RCA / 1st CID to replace their 6 pdrs and 17 pdrs with 2 pdrs towed by Jeeps in October 1944. Given the difficult terrain in Italy the lighter gun's increased mobility was seen as a definite improvement.

Mark 1, thank you for those very educational posts!

But the projectile can be a LONG way away from the chamber and still be in the barrel. It takes time for the pressure wave from continued expansion of the gases (continued burning of the powder) to reach the base of the projectile.

I'm not sure what you mean about 'a long way down the barrel'. The pressure wave will reach the base of the projectile and some of the gas will pass by it. This passby gas reaches the end of the barrel before the projectile. One small factor of round to round inaccuracy is the projectile has to pass through this gas when it slows down when it exits the barrel before the projectile and becomes turbulent.

Fascinating!

This is the sort of discussion that makes me keep wanting to read TMP. Where else would you get this sort of detail free, gratis and for nothing!

Very interesting, thanks very much Mark1

I'm not sure what you mean about 'a long way down the barrel'. The pressure wave will reach the base of the projectile …

Yeah, my words might not have been as clear as I wanted them to be. I am trying to articulate the physics based on my understanding, but without the actual numbers and equations in my hand.

So let's try it again.

The barrel of a Panther tank (for instance) was a little more than 17 feet long.

By the time the projectile was 80% of the way down that barrel (a bit more than 14 feet), it was travelling at more than 3,000 feet per second. (Actually more than that, but that number makes the arithmetic easier, so let's just go with that for the sake of illustrating the point.)

So that projectile that is 80% along, a bit more than 14 feet down the barrel, is going to leave the barrel (travel the last 3 feet) in less than 1/1,000 of a second.

In effect you have a cylinder of empty bore space, empty volume, that is 75mm in diameter by 14 feet long that has been created behind the projectile. The initial pressure generated when the projectile was in the chamber has been largely dissipated in this cylinder of empty space. If all the propellant powder burned in an instant, any reasonably containable pressure generated in the chamber would be dissipated by the expansion of this bore volume.

So the cartridge has been loaded with a large volume of relatively slow burning powder, to continue burning, continue generating expanding gasses, as this cylinder of bore volume grows, in order to keep the pressure up on the base of the projectile.

But that chamber, with that burning propellant, with those expanding gasses that are keeping the chamber pressure UP, is now 14 feet away from the base of the projectile. And the projectile will leave the barrel within 1/1,000 of a second.

So any pressure generated in the chamber at this time would have to travel 17 feet in less than 1,000 of a second to give ANY incremental push to that projectile. That means that the pressure wave would be travelling at more than 15,000 feet per second.

Pressure waves don't go that fast. Can't go that fast. No pressure wave travelling through air (gas), even compressed air, moves that fast.

What happens is that the pressure on the base of the projectile falls off dramatically as it moves up the barrel, even though the chamber pressure remains relatively high. The chamber pressure generated by the burning remainders of the powder does not equal the pressure on the base of the projectile. It takes time for the pressure from the burning remainders to reach the projectile, but given the distance and the speed of the projectile that can't happen while the projectile is in the barrel.

And so the last 20% of the barrel does not add to the muzzle velocity of the round. And the last 10% or so actually decreases the muzzle velocity, as the friction is still high, but the pressure (on the base of the projectile, not in the chamber) is not high enough.

At least that is how I have come to understand it. I hope I have done justice to the physics, even though I have not, in this case, provided the actual numbers and equations (which I have not retained).

-Mark
(aka: Mk 1)

Cool, so if you filled the barrel with water and made everything waterproof??

And so the last 20% of the barrel does not add to the muzzle velocity of the round. And the last 10% or so actually decreases the muzzle velocity, as the friction is still high, but the pressure (on the base of the projectile, not in the chamber) is not high enough.

75mm in diameter by 14 feet long that has been created behind the projectile. The initial pressure generated when the projectile was in the chamber has been largely dissipated in this cylinder of empty space. If all the propellant powder burned in an instant, any reasonably containable pressure generated in the chamber would be dissipated by the expansion of this bore volume.

14ft x 304.8mm = 4267mm/75mm = /L56.9

Did you know the German Paris gun was 211mm/L170?

How did the projectile even get to the end of the barrel let alone reach 81 miles distance?

From Wiki:

The longitudally perforated or multi-perforated cylinders used in large, long-barreled rifles or cannon are "progressive-burning;" the burning surface increases as the inside diameter of the holes enlarges, giving sustained burning and a long, continuous push on the projectile to produce higher velocity without increasing the peak pressure unduly. Progressive-burning powder compensates somewhat for the pressure drop as the projectile accelerates down the bore and increases the volume behind it.)

So it is not instantaneous but lasts a short time. The blast wave at some time +t does not have to reach 14' to push the projectile when it is at 14'. It just has to reach 1" before the projectile reaches 14'1" and the pressure in the 14' of barrel remains the same or increases.

Did you know the German Paris gun was 211mm/L170?

How did the projectile even get to the end of the barrel

A fascinating subject, the Paris gun!

Did you know that the last 6 meters of the barrel were smooth-bore? This portion imparted significantly less resistance to the projectile's passage. If the full length had been rifled it would have decelerated, rather than accelerated, the projectile.

Which brings us back to our original topic with an interesting aside. Somehow Gerlich (the original advocate of the squeeze-bore) managed to convince himself, and everyone he talked to, that the way to increase velocity was to make it HARDER for the projectile to travel down the barrel. Contrary to all that Gerlich preached, modern high velocity guns take the opposite approach, because you always get better velocity if you make it EASIER for the projectile to travel down the barrel (within the constraint that there must be enough of a seal that the propelling gasses push the projectile rather than just going around it).

The other interesting fact about the "Paris Gun" was that it was a 211mm caliber gun, built by putting inserts into old 380mm caliber breaches and barrels. So you had a 211mm gun with MORE chamber and barrel thickness than 380mm guns. It was of enormously heavy construction. That was needed, as the chamber pressure was enormous.

If your initial pressure wave is powerful enough, you can continue to accelerate your projectile over longer barrel lengths, as even though your "push" dissipates, it is dissipating from such a high value that it is still pushing more than the resistance of friction and rifling.
This is one of the reasons that smaller bore guns still sometimes have lengths greater than L55. You can afford more chamber pressure (relative to projectile mass) in a smaller gun. In a larger gun the weight of the gun quickly becomes unmanageable.

Still, if you have unlimited mass for the gun, and unlimited propellant, you can asymptotically approach the speed of a pressure wave in air, just by making your initial pressure wave so powerful. But the breech and barrel must be prohibitively massive.

The Germans seem to have had some understanding of this, as their super gun of WW2, the V3, used a "herring-bone" approach to propelling charges. As the projectile accelerated during firing, additional charges, layed out like a fish's ribs, fired all along the length of the barrel to keep the pressure up on the base of the projectile. This type of gun was able to hit London from the Pas de Calais on the French coast, but the gun complexes were so large that they were obvious targets, and were bombed into oblivion by the RAF.

The blast wave at some time +t does not have to reach 14' to push the projectile when it is at 14'. It just has to reach 1" before the projectile reaches 14'1" and the pressure in the 14' of barrel remains the same or increases.

Ah Mobius, my friend, you don't seem to give proper attribution to the nature of a pressure wave in air. Air is compressible. What happens at one end of the cylinder of air only affects that end of the cylinder of air, until there is enough time for the effect to propagate.

That is why hydraulics are different from pneumatics.

If you add gasses from incremental burning in the chamber when the projectile is at 14', the pressure will increase ONLY at the point of burning at that instant. It will take time for the pressure to increase 1" away. At the instant when the pressure goes up 1" away, it has NO effect on the pressure 2" away, much less 14' away. It takes time for the pressure to increase 2" away, and again more time to increase 4" away, and so on.

It takes time for that wave of pressure to move up the cylinder of air. In the end that wave of increased pressure will get to 14', and even 17', but by that time it will be a substantially lower incremental pressure, and it won't actually matter as the projectile will long-since have left the barrel.

Actually there will be no "increased pressure" from the residual burning of powder. All there will be is a slower decrease in pressure. But the principal is the same. The impact of slowing the pressure drop-off will take time to reach down the barrel. So pressures will drop-off much faster, the farther you get from the chamber.

Or so I understand. I make no claim to the revealed wisdom. It is an interesting subject, and I am happy to learn if my understanding if flawed.

-Mark
(aka: Mk 1)

It takes time for that wave of pressure to move up the cylinder of air. In the end that wave of increased pressure will get to 14', and even 17', but by that time it will be a substantially lower incremental pressure, and it won't actually matter as the projectile will long-since have left the barrel.

I think this is where you are going a bit wrong. You are saying that the pressure will drop because the barrel space the gas expands into has increased and on the other hand the space near the burning charge doesn't matter if the pressure there is increased. I maintain if the pressure is increased near the burning charge (when the projectile is 55+ calibers down the barrel) it acts as if the barrel space does not increase because the previous released gases cannot expand back into this newly pressurized space.

I maintain if the pressure is increased near the burning charge (when the projectile is 55+ calibers down the barrel) it acts as if the barrel space does not increase because the previous released gases cannot expand back into this newly pressurized space.

Fair enough. Can't fault that reasoning.

Still, in the pre-WW2 thinking, chamber pressure was largely what mattered. But once you got your projectile up to some reasonably high velocity (over about 3,000fps / 1,000mps), barrel lengths greater than about L60 just don't add to your velocity, because there is too great of a difference between your chamber pressure (which you have been measuring and working to increase) and the pressure on the base of the projectile, which is far away and moving fast.

And back to the original question, making a well-designed and balanced barrel longer AND increasing the resistance to the projectile's progress by adding a crushing force in addition to the friction and spin forces of rifling (as a Littlejohn adapter did) was finding 3 ways to slow down the projectile, instead of just 1 or 2!

Somehow we have some intuitive sense that the round should "pop" out of the barrel at higher speed because of all the pressure building up behind it from the squeeze. But that just isn't so. The pressure was not building, it was decreasing, and all the barrel extension was doing was slowing down the projectile.

(OK, not entirely all. It was also making the round retain it's velocity better at long ranges. But no one used a 2pdr for long-range shooting, so what did that matter?)

-Mark
(aka: Mk 1)

Very interesting discussion. I don't know if this helps but there is a graph in "Armoured Firepower" by Peter Gudgin (page 37) which shows the muzzle velocity as a function of barrel length in calibres. It shows the muzzle velocity falling off above 70 barrel length and flat above 90. It's not clear if this is based on experimental data or theoretical calculations and what diameter it is based on. The reference quoted is Nachrichtenblatt der PzTruppen,Nov 1944.