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OQF 6Pounder mk5 to 75mm

August 17 2012 at 4:36 AM
ian foster  (Login wompmug)
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from IP address 89.168.141.224

I have read that the British 6 pounder was rebored to accept 75mm ammunition.When comparing AFV Club's 6 pdr mk5 barrel with RB Models 75mm the former is the same length but slimmer. I believe this is correct.The diameter of the 6pdr mk3 barrel is similar to the 75 mm barrel but shorter.Can anyone shed any light on the subject,I must have got something wrong here!

 
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(Login BrianInOregon)
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50.53.229.250

If you're talking about the Cromwell, there are TWO 6-pounder barrels - Mk 3 & Mk 5...

August 18 2012, 5:48 AM 

The 6 pounder Mk 3 had a barrel tube length of 96.2". Overall length 100.95". Weight 768 lbs.

The 6 pounder Mk 5 had a barrel tube length of 112.2". Overall length 116.95". Weight 720 lbs.

The 75mm Mk V and VA had a barrel tube length of 107.8". Overall length, including muzzle brake, is 112.58". Weight 692 lbs.

This observation is made:

"The 6pdr Mk was distinguished from the Mk 3 by its noticeably longer barrel of "lighter" appearance. It was usually fitted with a counterweight on the muzzle."

(Reference: "British and American Tanks of World War II" by Peter Chamberlain and Chris Ellis.)

This seems to indicate that not only was the 6 pounder Mk 5 barrel longer, but it was smaller in diameter. This is born out by comparing the weight. The longer barrel weighs 48 lbs LESS than the shorter barrel. The only way this is possible is if its diameter is smaller.

Supposedly the 75mm Mk V barrel is simply a bored out and lengthened 6 pounder Mk 5 barrel.

I pulled out two sources, perhaps imperfect, to make comparisons.

From the 1/35th Cromwell I plans in "Cromwell Tank - Vehicle History and Specification" by The Tank Museum, the 6 pounder Mk 3 barrel has a diameter of 3.65mm. That equates to 5-1/32".

The 1/35th Tamiya Cromwell IV 75mm barrel is 3mm in diameter. That equates to 4-1/8".

I believe both dimensions to be in the ballpark, and support the above data.



    
This message has been edited by BrianInOregon from IP address 50.53.229.250 on Aug 18, 2012 5:53 AM


 
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ian foster
(Login wompmug)
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88.108.1.194

still bemused

August 18 2012, 9:41 AM 

Thanks for your answer Brian.From what you say it would seem that the 6pdr mk5 tube is slightly longer than that of the 75mm gun employed in the Churchill tank and the 6pdr mk3 tube is shorter but of a more suitable diameter.I am begining to wonder if rebored 6dr means employing the basic 6pdr mk 5 pattern but with a slightly increased bore and andexterior diameter,or even aconsiderably extended 6pdr mk 3 barrel?

 
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Claus Bonnesen
(Login cbo)
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Re: OQF 6Pounder mk5 to 75mm

August 18 2012, 11:56 AM 

Calling the 75mm MkV a rebored 6-pdr is a bit of a stretch. It would be more corret to call the 75mm MkV a gun built around the internal ballistics of the US 75mm M3 gun using as many 6-pdr parts as possibly and able to fit in the 6-pdr gun mount.

According to the Churchill Mk VII & VIII Instruction book, the 75mm barrel was "externally similar to that of the Ordnance Q.F. 6-pr 7-cwt. Mk V" and measured 107.8 inches. As Brian points out - and confirmed by the Instruction Book for the Cromwells 6-pdr gun - that was shorter than the 112.2 inches of the 6-pdr Mk V barrel.

My guess is, that the difference in length and weight is found in the breech end of the gun, as 6-pdr cartridge was longer than its 75mm counterpart (441mm vs 350mm).

As for the thickness of the barrel, it stands to reason, that a barrel with thicker walls was used for the larger 75mm round.

 
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(Login BrianInOregon)
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Gun balance...

August 18 2012, 3:49 PM 

It seems logical that the barrel walls should be thicker for the 75mm vs the 57mm/6 pounder, but is that really the case? And then there is the problem of gun balance.

The British did not use elevation gears for the guns in this discussion. The guns were balanced with a slight bit more weight to the breech end. The gunner would move the gun up and down manually to compensate for elevation. This has advantages for firing on the move in pre-gyroscope days, though it is an obvious drawback for sustained indirect fire.

The weight of the 75mm vs the 57mm/6 pounder Mk 5 would have to take into consideration not only the larger bore, but also the larger chamber. It's possible that the gun tube was left at the same exterior diameter because of a balance issue. The gun tube is longer and has a muzzle brake on it as well.

As for the 75mm shell requiring a heavier tube wall, well, I did some checking on 1/35th plans and models. The M2 and M3 guns used by the M3 Grant/Lee and M4 Sherman are considerably thicker than the 75mm Mk V. However, the 75mm M6 gun used on the M24 Chaffee is, by measurements of the plans, models and aftermarket barrels, slightly smaller in diameter than the 75mm Mk V gun.

So from a pressure standpoint, it appears a smaller gun tube is OK for this shell. So why the heavier tube on the M2/M3? First, to balance out the much heavier breech. Second, heavier walls remain more accurate for a longer time period when heated up from sustained firing.


 
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Claus Bonnesen
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Elevation gears

August 19 2012, 4:08 AM 

According to David Fletcher in "Universal Tank", the 75mm required an elevating gear. When installed in the Churchill, it had elevating gears according to the Instruction Book. Descriptions of the 75mm armed Cromwell also speaks of an elevating gear and I belive the Valentine had geared elevation as well, when armed with the 75mm ("Universal Tank" and other sources).

I think the important bit here is, that the 75mm Mk V was not simply a "rebored" 6-pdr, but a new gun, developed using 6-pdr parts. So it could very well be that the barrel of the 75mm differed from that of the 6-pdr Mk II and V as per Ians original question. Another source (old AFV Profiles article on the Cromwell) says of the 75mm: "The 75-mm used many parts in the breech mechanism which were identical with the 6-pdr gun. That barrel was similar...."

As for the actual thickness of the barrel, a wartime source or an actual measurement of the real thing is probably what is needed to confirm the realiability of the models Ian speaks of.



 
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ian foster
(Login wompmug)
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physical evidence

August 19 2012, 4:45 AM 

Claus is probably right, in that taking actual measurments of the diameter of the barrels in question would provide a definative answer,anybdy have a couple handy?

 
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(Login BrianInOregon)
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Absolutely... Unfortunately...

August 19 2012, 4:52 PM 

Unfortunately, no one has stepped forward with actual measurements, which is why I went out on a limb with my data and opinion.

If anyone is near a museum vehicle, measurements would be appreciated.

And that's interesting info on elevation gears for the 75mm Mk V. None of my references mentioned it used gear elevation.


    
This message has been edited by BrianInOregon from IP address 50.53.229.250 on Aug 19, 2012 5:36 PM


 
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ian foster
(Login wompmug)
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88.108.5.13

Thanks all

August 22 2012, 3:50 AM 

Interesting discussion,I think Bovingdon tank museum could be the way to go, I will try to make a visit.

 
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