76 mm vs. 75 mm guns on Shermans

[fatherof4]

Why was the 76 mm gun on Shermans so much more effective than the 75 mm version? That extra 1 mm doesn't seem like it would make very much difference.

Was the newer gun barrel significantly longer? I know (from reading this board) that longer barrel = faster shell = more penetration but have never seen a discussion of exactly why the 76 mm was better.

Thanks,

fo4

[risc]

I don't think Sherman's 76mm gun was so much better than 75mm. It was better but not that much, personnal opinion of course. Though the gun was better against Mark IV, it still lacked against , Panther or Tiger for long range engagements...

Peter

[dano6]

The difference between the 76mml53 and the 75mml40 is all in the muzzle velocity derived from the added barrel length and a slightly longer casing for the 76mm gun. The armor penetration of the 76mml53 gun was much higher than that of the 75mm pea-shooter. While the 76mm could easily penetrate the pz4 at long range, it still could not kill the panther with front shots at over 200-400m without APCR (HVAP) ammo. I do not have muzzle velocities available to me now, so I will leave that to another poster.

As for effectiveness, 76mm penetration at ranges over 500m was significantly higher than the 75mm and the abitilty to use APCR ammo also made the gun much more effective against the german heavy tanks. Thus making duels with pz.IV (the most abundant german tank by the way) equal if not better in favor of the sherman especially if the sherman had added welded on armor.

dano6

[This message has been edited by dano6 (edited 09-09-99).]

[Dar]

I would agree that the extra 1 mm wouldn't make a whole lot of difference. However, there must have been some increase in the velocity if the 76mm were dramatically more effective than the 75mm.

Note that the German Panther's gun (75L70) was much more powerful than the PzIV's gun (75L48), despite both being 75mm. Likewise, the King Tiger's gun (88L71) was more powerful than the Tiger's gun (88L56). Each gun compared was of the same caliber, but the barrels were longer in the more powerful versions, and, I'd be fairly certain, the shell had more propellant.

Dar Steckelberg

[dano6]

If I remember correctly the 75mmL40 gun had a muzzle velocity around 2300 fps, whereas the 76mmL53 gun had a muzzle velocitty around 2800-2900 fps. And with the APCR ammo it was around 3200 fps. Please correct me if these are wrong as I am going from memory.

Also remember that the higher the velocity the flatter the trajectory and the chance to hit the target. This is why the 76mm gun was much more effective as an anti-tank gun.

The shell casing of the 76mm was only maybe 1 cm longer than the 75mm. The major increase came from the 13 calibers longer of a barrel on the 76mmL53 weapon compare to the 75mmL40. Remember the L40 designation refers to the barrel being 40 calibers long. That would mean that 40X40mm would equal barrel length. So therefore the 76mm weapon had a longer barrel and thus a higher muzzle velocity.

dano6

[fatherof4]

So for the non-grognards out there like myself, the 76 mm barrel was 76*53 = 4.03 m (13.2 ft) long while the 75 mm was only 75*40 = 3.00 m (9.84 ft).

That's a 34% increase in length, which does sound significant. And, if I understand it right, the reason longer is better is that more of the explosion from the propellant is transferred to the shell.

Thanks for your help guys.

[Guest Big Time Software]

Gun caliber is not the be all end all measurement, as has been explained above. Instead, it is just one part of a larger set of important stats. Kinda like CPU clock speeds. By themselves they only give you ballpark ideas about how fast the machine is.

The extra length also gives the round more of a chance to spin, which increases range and accuracy.

Steve

[dano6]

Actually Steve, in the above example, the main contributing factor to higher muzzle velocity and therefore effectiveness is the longer barrel. Both AP casings for both the 75mm and 76mm are very similar as are the rounds(mass and shape). This would lend very little difference to the powder charge or the projectile. Therefore to the actual propellant force of the charge would be very similar. The longer barrel keeps the expanding gas and the force of the expanding gas in the barrel longer thus moving the projectile at a greater velocity. This means the pressurized gas performs more work on the projectile in a longer barrel.

As for effectiveness and accuracy the longer barrel also helps here. The added velocity means a flatter trajectory to the target giving a higher percentage of hitting. This is achieved because there is not as much adjustment in elevation for different ranges and range determination is not as important. Also the higher velocity means more armor penetration if AP round design and weight is equal. The extra spin derived by the longer barrel also helps in longer range accuracy(1000m+). This is because the small amount of extra spin gives the round a little more stability through the air and helps with longer range accuracy.

I know that all of this is known by you, but the main difference between the 75mm and the 76mm is actual barrel legnth and therefore higher muzzle velocity.

[Guest Big Time Software]

Yes, my goof. I meant to say "shell caliber" is not the be all end all measurement. I said "gun caliber" instead, totally forgetting that barrels are ALSO measured in calibers (for what stupid reason I will NEVER understand ). As you say, the single most important aspect of any firearm/gun is the length of the barrel with talking range, velocity, accuracy, and power. Think of a snubnose 38 versus a long barrel revolver with the same round. Same concept.

Steve

[Simon Fox]

I actually find the British designations (2pdr, 6pdr, 17pdr) intensely annoying when it comes to this sort of discussion. While I have a fair idea as to where they fall in the scheme of things I would be interested in knowing what the characteristics of these guns are when expressed in the standard nomenclature (ie 75mmL40). I am sure that someone can satisfy my curiosity.

[Pak40]

Some more factors to consider in muzzle velocity:

1) Muzzle Break - I'm not sure why but I've read that guns with a muzzle break do have a higher velocity. Some US 76mm had muzzle breaks didn't they?

2) Tapered Bore - American guns didn't have them but I think some German guns did. Maybe Steve can fill us in on the physics behind these guns.

Simon,

I don't know the lengths of the barrels but I know the size of the shells of British guns:

17lb - 76mm (actually something like 76.2mm) It was longer than the US 76 and had a muzzle break.

6 lb - 57mm

2lb - 37mm I think

[Fionn]

Simon:

This is an interesting point actually. There are NO standard conversions for british rounds simply because they talked about the weight of the shell AND as has been shown here before it is quite possible to have the exact same 88mm shell fired from both an 88l/56 and an 88 L/71.

Of course that is the theoreticall correct answer. In practice only a few different guns firing 17 pounder, 6 pounder etc rounds were made so we can give a rough idea of the calibre of these guns.

E.g. 6 pounder = 57mm shell BUT there were at least 5 versions of the 6 pounder gun ranging all the way from a 57mm L36 to a 57mm L43 in the most common versions issued to troops during the war (the Mk III and Mk V respectively).

17 pounder refered to the 76.2mm L/55 which fired a 17 pounder shell BUT the shell fired by the 76.2mm L49 ALSO weighed 17 pounds.

Interestingly, to differentiate between the 76.2mm L/49 and the 76.2mm L/55 which actually used differently shaped ammunition the 76.2mm L/49 (which was used on the Comet) was officially entitled as a 77mm gun.

The reason for the 76.2mm L/49 being designed was that it was purpose-built to put into the Comet cruiser tank which wouldn't have been able to handle the recoil of a full 76.2mm L/55.

The reason for designating the 76.2mm L/49 as a 77mm gun was simply to ensure that logistics and supply personnel wouldn't get confused.

It's the same system as used by the Soviets where tank main guns are 125mm, artillery use a 122mm calibre and mortars a 120mm calibre. By varying the bores OR by simply incorrectly designating the bores you can ensure that tank rounds won't be delivered to an artillery battery and vice versa.

Same thing happened with the 76mm gun in use on light Soviet tanks (e.g PT-76) and the 73mm gun as first put on the BMP-1. 73mm is a VERY unusual bore for a gun BUT it ensured that poorly educated supply clerks wouldn't mess up during a war. A plethora of similar examples can be found throughout the Red Army (I used to be very interested in the Soviet Army so that's where all this comes from. )

Hope that helped a little.

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Fionn Kelly

Manager of Historical Research,

The Gamers Net - Gaming for Gamers

[Fionn]

Pak 40:

Tapered Bore (otherwise known as Gerlich guns or "Squeezebore" ) were utilised by several nations.

The venerable 57mm AT gun in use by the Americans had a tapered bore adapter as did several German guns, most famously the 28mm Panzerbusche 41 (one of my favourite "cool idea" weapons) also used a tapered bore:

In its case the Panzerbusche 41 squeezed the round down in calibre from a 28mm round (initial) to a 20mm round (emergent).

A 42mm AT gun also featured tapering down from 42 to 28mm and several adapters were developed and fitted to other AT guns but I'm not sure they were very common at all.

The British 6-pounder used a "littlejohn adapter" to squeeze the round down also.

End result of all this tapering was that the rounds exited with a higher velocity than they otherwise would and since the necking down of the round also resulted in an increase in round density (always nice to have when playing armour penetration games) you ended up with a round of higher density travelling at a much greater velocity and less subject to the vagaries of wind resistance.

All in all a nice thing to have happen EXCEPT that tapering the rounds quickly wore out the rifling on the barrels (this was before the days of smoothbore) and wore the AT gun out much more quickly than normal. Hence it wasn't very popular in most armies EXCEPT where the need for mobility necessitated a small bore AT gun..

With a small bore AT gun about the only way to increase its velocity sufficiently was to taper bore it.. That is why the Panzerbusche 41 is man-portable and a really effective light tank killer.

I did a lot of this research on this for patches for another game on the market so feel free to keep firing away . I LOVE dealing with all this tank-related stuff .

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Fionn Kelly

Manager of Historical Research,

The Gamers Net - Gaming for Gamers

[Doug Beman]

The Germans messed about with a tapered bore gun, designed to fire a special kind of tungsten-core shot. The squeeze would both decrease the diameter of the shot (increasing "punch-power" by making a small superhard dart) and, by decreasing the volume of the bore, increase gas pressure for even higher muzzle velocity.

The shot had to start out as the standard size (eg 50mm, 55mm etc) in order to keep the propellant load the same and allow ammo standardization (otherwise the guns would just fire a tiny dart; there would be almost no propellant for gas pressure)

The Germans discontined the guns due to lack of tungsten, the difficulty of manufacturing the barrels, and the increase in procurement/resupply workload.

DjB

[Herr Oberst]

Hmm, this one's a REAL stretch, but I think I remember reading that the muzzle brakes, allowing some of the propellant gasses to escape out the side not only lessened recoil, but also negated part of the effect of a vacum behind the round as it exited the barrel. Cleaner exit and less destabilizing vortex behind the round.

Perhaps a real einstein on the matter can tell me if I am remembering correctly, or should I just flush this concept out of my brain...

[Rhet]

<BLOCKQUOTE>quote:</font><HR>...you ended up with a round of higher density travelling at a much greater velocity and less subject to the vagaries of wind resistance.<HR></BLOCKQUOTE>

Fionn, very good explanation. I would only add two more benefits gained by the tapered bore systems. These would be reduced impact area and increased axial inertia.

for a better explanation I will use the 42mm/28mm (PaK 41)AT gun. Muzzle Velocity=1250m/s AT round mass=.31kg

In essence the round's cross sectional area is reduced from 1385mm^2 (@42mm dia.) to 616mm^2 (@28mm dia). Thus the emergent cross sectional area of the round is only 45% of the original area. This reduced cross section translates into more hitting power by virtue of the fact that the round simply has to displace less armor to penitrate.

The second benefit I mentioned above is more subtle than the first. As the round is squeezed down from 42mm to 28mm it deforms and grows longer (along its long axis). As a result it's axial inertia increases. This has two substantial benefits. The first one as you mentioned is improved flight characteristics. The second is better performance against sloped armor. As the round contacts sloped armor its higher axial inertia causes it to resist deflection better. More simply the round "bites" more heavily on the sloped surface. This is one of the main principles behind todays "long rod penitrator" rounds.

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Rhet