Big Guns aloft in WW2

ThomasP said:

The 2pdr/40mm 'S' gun APCNR projectiles fired with the Littlejohn adapter had tungsten-carbide cores. The type of tungsten-carbide used had a density of .48 lbs/in3, and was quite hard, but not as brittle as the usual tungsten-carbide cutting tools of the time. I do not know if both types of projectile were trialed, or only one.

View attachment 617725

Click to expand...

I have only ever seen one photo of a 40mm Class S Littlejohn round, and that showed the Mk II projectile on the right.

Thanks for the info.

I read that the Littlejohn adaptor was hardly ever used. It prevented any other type of ammunition being used and firing the Littlejohn rounds without the adaptor at the shorter ranges armoured cars fired at meant little difference in penetration.

Littlejohn adapter would've made most sense exactly on tank-busting aircraft, since the ammo type is just one to be loaded with.

fastmongrel said:

I read that the Littlejohn adaptor was hardly ever used. It prevented any other type of ammunition being used and firing the Littlejohn rounds without the adaptor at the shorter ranges armoured cars fired at meant little difference in penetration.

Click to expand...

My Yeomanry predecessors used the Littlejohn rounds but without the adaptor in their Daimler armoires cars. They felt the penetration was enough for anything they could penetrate and it allowed them to use the HE rounds at will.

yulzari said:

My Yeomanry predecessors used the Littlejohn rounds but without the adaptor in their Daimler armoires cars. They felt the penetration was enough for anything they could penetrate and it allowed them to use the HE rounds at will.

Click to expand...

From what I have read the effects of the squeeze bore didn't make much difference until the range was over 400 yards or so. I imagine the 2 PDR on an Armoured Car was more of an "Oh shit", shoot and scoot gun no one would be firing carefully aimed shots at long range.

A Littlejohn round is going to act very much like like an APCR (Armor Piercing Composite Rigid) with out the muzzle adapter and a lot closer to an APDS round with the adaptor.
Little difference in AP performance until the range opens up enough for the superior ballistic shape of the squeezed down projectile to show it's advantage. The muzzle adapter my give a bit higher initial velocity.

Penetration for 2pdr tank and AT gun with/without Littlejohn adapter:

Just a tad flatter trajectory ...

Shortround6 said:

A Littlejohn round is going to act very much like like an APCR (Armor Piercing Composite Rigid) with out the muzzle adapter and a lot closer to an APDS round with the adaptor.
Little difference in AP performance until the range opens up enough for the superior ballistic shape of the squeezed down projectile to show it's advantage. The muzzle adapter my give a bit higher initial velocity.

Click to expand...

I would have expected the Littlejohn adaptor to act as a squeeze gun significantly increasing the MV and with that better penetration at shorter ranges.

I guess it depends on what "significantly increasing the MV" means. An extra 3-400fps on a 4000fps projectile or perhaps an 600fps on a 3800fps projectile?

With the Little John on a 2pdr AT gun (not aircraft gun) the projectile accelerated down the bore the full distance with the pressure in the bore dropping just about all the way (peak pressure may have been just a few inches in front of the chamber.) Then the projectile hits the tapered section, now, does the pressure in the barrel (pounds per square in of shell base area)go back up? I doubt it. The powder is pretty much all burned at this point, volume in the bore is still going up, Not like the projectile exiting into open air and not the increase in volume you would get with a longer conventional barrel. So you are just keeping the pressure from falling off as quickly as a longer parallel wall barrel. This will increase velocity, the question is how much.

The German Taper bore guns, at least the smaller ones, started their tapers much closer to the chamber when the pressure was higher so they could maintain (or at least suffer a slower fall off) the higher pressure over a longer period of time. A slower increase in the volume of space in the barrel as the projectile traveled down the barrel.

The Little John is a careful balancing act. The Pressure in the barrel cannot be too high when the projectile hits the taper or the barrel/adaptor will be bulged if not blown off the end of the gun. On the other hand it cannot be too low or there will be no advantage. The pressure may be 4-6 times (guessing, conservative) lower when the projectile hits the adaptor then it was at point of peak pressure with projectile several inches down the barrel.

Reducing the diameter of the projectile is a considerable advantage as a 0.93lb projectile trying to make a 40mm hole in the air is going to slow down rather quickly.

I don't believe that the Littlejohn adapter added any velocity. In fact, I suspect that the added friction once the projectile reached the adapter might well have slowed it down by comparison with firing the squeezebore shot without the adapter (although of course the aerodynamic drag would be much worse, limiting effective range).
Pressure increase when the shot hit the adapter seems to have been a problem, as the adapter has pressure-relieving ports:

At roughly what range does the squeezed projectile start to have an advantage over the unsqueezed projectile.

About 5 yards

The unsqueezed projectile slows down faster so the advantage increases with range.
Reliably measuring the difference at close range may be difficult.
Noticing the difference field?
Armored cars should not be taking long range pot shots at large tanks to begin with.

The A-26B/C could be fitted with a 75mm cannon, but I don't know if that was ever used operationally.

The comments from my forebears were that, if the naughty people were far enough away to need the aerodynamic advantage of the squeezed round, then you are better off running away firing off all the smoke you can make. Daimler's go fast in reverse. The HE was mostly used for reconnaissance by fire. I.e. lobbing HE rounds about until someone moves or fires back. Advice was not to take on an AT gun in a duel with 2 Pounder HE if you can avoid it. He will probably win. Humber's and Staghounds carried 37mm canister up the spout to immediately react to naughty infantry and Panzerfausts close up. Meanwhile the commander is shoving in the chosen next round type. I doubt if any of this was in a formal document.