What does "L/xx" tell me?

[GreenAsJade]

I presume it's the length of the tank's gun. Does this information tell me something about the performance of the tank, or is it for interest only?

[Herr Kruger]

Generally speaking, the longer the gun the better antitank ability it has while shorter guns are often good for attack against soft targets only. But I'm not a grog.

J Kruger

[Shosties]

It's the cannon's 'caliber' which in this instance refers not to the bore diameter, but the length of the barrel in terms of a multiple of the bore diameter.

Thus "7,5 cm PaK 40 L/46" gives you:

75mm bore diameter. Barrel length of 75mm x 46 = 3450 mm (3.45 meters).

To launch shells with higher velocity, you need to burn more propellent, which takes more time, so lengthening of the barrel is necessary to provide more "working distance" (like increasing the piston stroke of an engine).

[John D Salt]

Originally posted by Shosties4th:

It's the cannon's 'caliber' which in this instance refers not to the bore diameter, but the length of the barrel in terms of a multiple of the bore diameter.

The usual term is "calibre length".

The US convention was to measure the length of the bore only, the German convention to include the length of the chamber. I imagine that this explains why the 75mm M3 gun in the Sherman, known to generations of wargamers as an L/40 piece, is listed in CM:AK as an L/38.

All the best,

John.

[Sergei]

Originally posted by Shosties4th:

To launch shells with higher velocity, you need to burn more propellent, which takes more time, so lengthening of the barrel is necessary to provide more "working distance" (like increasing the piston stroke of an engine).

Good answer. Another example would be a blow-pipe. For a pea or whatever to reach high speed, you have to blow hard, but if the pipe is too short the pea will just fall out of the pipe's end. Also I think one effect of a long barrel is that it stabilizes the round and thus improves predictability of the pea's trajectory. Added speed makes long range shooting more accurate as such. I think everyone who has lived a childhood at some point of his life knows this!

[CombinedArms]

Originally posted by Sergei:

[snip] Another example would be a blow-pipe. For a pea or whatever to reach high speed, you have to blow hard, but if the pipe is too short the pea will just fall out of the pipe's end. Also I think one effect of a long barrel is that it stabilizes the round and thus improves predictability of the pea's trajectory. Added speed makes long range shooting more accurate as such. I think everyone who has lived a childhood at some point of his life knows this!

Given the apparent advantages of longer barrels, why were most tank guns short barrelled at the start of the war? E.g. the PzIV didn't really become lethal till it got that long 75, but it took several years for that to happen. Similarly, the Sherman was slow to convert to a longer gun and never completely did so. There are obviously many, many other examples.

Was there a perceived advantage to shorter guns in the eyes of most designers (in all armies) earlier in the war? Or were short guns just cheaper and longer guns weren't deemed necessary? Or did technical problems with long guns in tanks have to be solved? I'm not sure if this is right, but it seems like the Tiger and Panther may have been the first tanks with proper turrets to be designed from the ground up with long guns.

[Sergei]

There would be some benefits from low velocity guns against infantry, as they can 'lob' their shells better over slight rises in the ground (do you remember the "un-hittable gun bug" in some versions of CMBB???). But the main reason was in the implementation.

Longer guns require a bigger explosive charge to actually work. This has several consequences: ammo takes more space, much larger recoil, more strain on the breech & gun, longer barrel - all these result in a bigger and heavier gun. All AFV's just couldn't be fitted with a heavy gun in the turret, the AFV had to be designed for this. That's why the Grant had that 75mm gun in the sponson and why Pz IV's were retro-fitted with long 75's but Pz III's with short 75's. Also why KV-2 ended up being such a monstrosity that couldn't rotate its turret when on a slope, because the turret wasn't balanced evenly. And it was also metallurgically challenging to produce high quality, high velocity guns - which raised the cost. For instance, Soviets did have very good 57mm HV guns available, but they just were too expensive to produce compared to the less effective 45mm ATG's. The 100mm guns were also too expensive for larger scale use despite their effectiveness compared to 85mm and 122mm guns.

And early in the war the role of tanks was still a bit unclear, and most tanks could be penetrated by the 37mm ATG. Usually there weren't enough of even those, so it was better to go with the sufficient and cheap than over-performing and expensive. The need for HV guns was only realized as heavier tanks were faced in combat - Char B1, Matilda II, T-34, KV, Tiger...

[flamingknives]

Another problem with long barrels is that they are more likely to be damaged as they stick out further.

The length also means that small deflections caused by temperature variations create a more significant inaccuracy. Modern tank guns often have a thermal sleeve to avoid this.

Last, and by no means least, longer guns are harder to make - you've got to have a lathe large enough and tooling rigid enough to machine the bore and the rifling. This increases cost and resources.

[GreenAsJade]

So a higher calbre length implies advantages more than just the increased penetrations that seem to be associated with it: also greater accuracy (which doesn't show up in any other table/parameter)?

[Redwolf]

Originally posted by CombinedArms:

Given the apparent advantages of longer barrels, why were most tank guns short barrelled at the start of the war?

It's not really true for the British, who has pretty long guns as the tank-killing weapon in their tanks.

For the Germans, the Panzer III with the comparably long 37mm on was tank to fight enemy tanks. The Panzer IV was for close infantry support. The Germans just drastically undereastimated how thick enemy tanks would become how soon, Valentine and T-34 were a bad suprise.

The Russian 45mm and 76mm guns on early war tanks are not that short.

The Americans just goofed.

[flamingknives]

Short barrels, like that of the 75L24 on the PzIV mean lower velocity, so the shell doesn't have to be as strong to widthstand firing stresses. This means that a greater proportion of the shell can be explosive, which is ideal for a support HE chucker.

An arcing trajectory is also to be desired for putting HE on target, AIUI.

[Other Means]

also, a HE shell fired at high velocity tends to bury itself in the ground, wasting a portion of the blast.

[CombinedArms]

Originally posted by redwolf:

The Russian 45mm and 76mm guns on early war tanks are not that short.

The perception from CMBB, though, is that these guns could be more lethal against armor--maybe the problem was ammo quality? Didn't the Germans make an AP shell for captured 76mm ATGs that vastly increased their effectiveness?

BTW, I'm fighting a desert scenario where my Germans have a captured 76mm ATG (very lethal.) Would this be a captured Russian gun? Did the Germans really transport them all the way to North Africa?

BTW, also--my perception is that, for it's comparatively short length, the Sherman 75mm is actually surprisingly effective. Is this a case of good AP ammo partly overcoming the limitations of the gun?

[Michael Emrys]

Originally posted by CombinedArms:

Would this be a captured Russian gun?

Yes.

Did the Germans really transport them all the way to North Africa?

No, they grew them in an oasis in the middle of the desert.

Michael

[CombinedArms]

Originally posted by Michael Emrys:

</font><blockquote>quote:</font><hr />

CombinedArms:

Did the Germans really transport them all the way to North Africa?

No, they grew them in an oasis in the middle of the desert.

Michael </font>

[Big Jim]

A combination of doctrine and production difficulties seems to be the general consensus- I suppose going on WWI, you wouldn't have much of an idea how armoured warfare would develop...

Does anyone know if the original Panther design before the invasion of Russia was going to be designed with the lethal 75mm L/70(?), or had they not got to that stage of design?

[Sergei]

Designing Panther only started after lessons learned with T-34.

[Michael Emrys]

Originally posted by CombinedArms:

</font><blockquote>quote:</font><hr />Originally posted by Michael Emrys:

</font><blockquote>quote:</font><hr />

CombinedArms:

Did the Germans really transport them all the way to North Africa?

No, they grew them in an oasis in the middle of the desert.

Michael </font>

[MikeyD]

Two instances of gun tubes getting shortened are the Sherman's 76mm tank gun and the Russian 57mm ZIS-2 anti-tank gun.

The Russian gun started the war at a phenominal L/86 calibers but the gun was horribly expensive to produce and would wear rapidly. When it was reintroduced in '43 it had a shorter L/63.5 barrel (if I read the "Russian Battlefield" site correctly).

I'm not near my U.S. gun references (Hunnicutt's SHERMAN) but I recall the 76mm gun initially had a much longer barrel and was shortened in order to be less cumbersome (unbalanced on the trunnions, liable to strike something while maneuvering, etc. ) I can't recall how much it was hortened - L/62 to L/52?

Ah! And the Comet got a shortened 17 pdr too!

[Big Jim]

Originally posted by Sergei:

Designing Panther only started after lessons learned with T-34.

True, the final form of the Panther was decided upon after encountering the T-34, with thick frontal armour, speed and a 75mm L/70 gun.

According to http://www.wwiivehicles.com/html/germany/pzkpfw_v.html however:

"Porsche was instructed at the end of 1939 to develop a heavy tank between 25 and 30 tons with the 7.5 cm KwK L/24 gun, and possibly later have the 10.5 cm KwK installed. Nicknamed the Leopard within Porsche. It had twin engines. Nibelungenwerke in St. Valentin built 2 prototypes in 1940.

When the Russian T-34/76 was encountered the design program was sped up. In November 1941, General Guderian (commander of Panzergruppe II in Russia) wrote a report suggesting that a commission should be formed immediately to design a tank to be able to gain supremacy over Russian T-34 tanks. The Armaments Ministry sent a commission to the Russian front to study the T-34."

Funnily enough they decided to up the gun size after meeting the T-34... I'm suprised Hitler didn't countermand the order so as not to lower the morale of the PzkwIV crews.

[Other Means]

can anyone give me an explanation i might understand about why the length of the barrel is best as a multiple of the caliber?

a tall order but there you have it.

[Sergei]

Maybe because it tells us more about the muzzle velocity. Because the ratio between caliber and barrel length is really the decisive thing here, not the absolute length of the gun.

[Other Means]

i'm sorry but how does it tell us anything about MV? and why is the ratio important rather than the absolute length of the barrel? why is 40 L/46 more accurate than 40 L/46.5. or is it not?

...i can see that a larger caliber shell will take more propellant to get it to move and therefore maybe the ratio between caliber, propellant & barrel length says something...but again why is the length a multiple of the calibre?

am i missing something obvious? like a clue? sorry to hijack the thread.

[Sergei]

See the post by Shosties4th:

"To launch shells with higher velocity, you need to burn more propellent, which takes more time, so lengthening of the barrel is necessary to provide more "working distance" (like increasing the piston stroke of an engine)."

Here are some gun stats from CMBB sorted by caliber length. I think you can see the dependence between it and MV:

88 L/71 1018 m/s (AP)

75 L/70 925 m/s (AP)

50 L/60 835 m/s (AP)

88 L/56 773 m/s (AP)

20 L/55 880 m/s (HE)

85 L/55 800 m/s (AP)

75 L/48 750 m/s (AP)

47 L/43 782 m/s (AP)

37 L/33 495 m/s (AP)

75 L/24 420 m/s (HE)

37 L/21 388 m/s (AP)

150 L/11 240 m/s (HE)

75 L/11 210 m/s (HE)

[Sergei]

Note that 75 L/24 and 150 L/11 have roughly the same absolute barrel length (180 cm vs. 165 cm), but the former has a MV almost twice that of the latter. Meanwhile 75 L/11 has a barrel only half as long (82.5 cm), but almost the same MV as 150 L/11.