Mysterious Markings on a 10 Pdr. British Screw-Gun

[seacoastartillery]

       We may need some help from our friends in the U.K. or Australia or New Zealand to solve the latest mystery uncovered by the seacoast boys.  While searching the internet aimlessly last night due to GBO forums being down, I happened across a photo of a 10 Pdr. British Mountain Gun with some curious markings.  I found out later, as I refined my search, that it was one of the guns now for sale at the Nepalese Armory guns importer, IMA-USA.  This fact makes the markings no less mysterious to us.

      Before we show the photos, you have to read a short paragraph of background info.  Originally designed by Colonel le Mesurier of Britain’s Royal Artillery in 1877, the first of these Mountain Guns were 7 Pdrs. with 2.5” bore dias.  The tube was made in two parts for ease of mule transportation.  They were sent to Afganistan only two years later and were very well received.  The rifling that imparted guidance to the little shells was gain twist, eight groove, from one turn in 80 cals. to 30 cals. almost all the way to the muzzle.  The last 4 inches were one turn in 30 uniform twist. 

      This same design with minor changes lasted into the 1890s when a breech loading screw gun was developed, with several versions.  The final was the 2.75”, 10 Pdr. bag gun, Screw Gun, fired by friction primers or friction quills if you prefer.  This is the gun on which we spotted the mysterious marks.  This is the gun currently sold by IMA-USA. 

      Since these screw guns were rifled and the rifled tube sections were screwed together before use, we are wondering how, with the rotational movement allowed for wear indicated by the arrows in the first photo, would this tolerance actually work.  We don’t think accuracy would survive  ANY  rifling mismatch at all and certainly NOT as much as is displayed between the arrows.  There must be an alternate explanation for these markings. Let us be clear; we don’t know.  We have our theories, but no real facts, yet.  How would you explain them??

Mike and Tracy

 
The Mysterious Markings at the junction of the breech section and the muzzle section of the 10 Pdr. Screw Gun.




The whole screw-gun.





The gain-twist rifling.  See any mismatch?  Looks like a form of 'Slant-Hook' rifling.

[A.Roads]

Hi Mike & Tracy an interesting question & one that I don't know the answer to.
However I believe that there are two likely possibilities:
 
1). Because the projectiles are lead covered & grip the rifling with a "squeeze" effect I suspect that they may be largely unaffected by the change in rifling alignment, by the time the shell exits the muzzle it is under the influence of the rifling in the muzzle section.
 
2). If there is an inner bore sleeve perhaps that could be unlocked, counter rotated, & then locked into its new position.
 
I think that 1). is the correct answer & when I am with my books I'll see if I can locate the reason.
 
That form of rifling is known as Ratchet rifling, it is usually associated with small arms & is an uncommon rifling type.

Regards
Adrian

[seacoastartillery]

      Thanks Adrian, we certainly appreciate your interest.  It’s very hard to believe that lead would be coating the projectiles at this late date, 1903, especially with the French 75, Modele de 1897’s shells having a prominent copper driving band.  Also the 3.2 Inch U.S. Field Gun M1890 had driving bands on it’s shells.   

    “It took a minimum of four mules to carry just the gun and mount. Two mules each to take half of the rifled barrel, one mule to carry the mount and a forth mule to carry the pair of 36" spoke wheels. Additional pack animals were required to transport the ammunition, which consisted of bagged powder and separate studded projectiles with quill-fused primers.”   From the web site www.ima-usa.com, the information given is correct except for the reference to ‘separate studded projectiles’ which is wrong .  Studded projectiles are used with deeply grooved rifling, definitely NOT the slant hook, or ratchet rifling shown in their gallery of photos for the BL 10 Pdr. Mountain Gun.   

     And now we are back to the Screw Gun.  This was the most advanced version, used widely in WWI.  The shell depicted is from a RA manual of unknown title and is noted as being issued in 1915. A more refined type of driving band is shown on it, very similar to that on the 37mm Hotchkiss shells used in revolving cannon in 1915-1918.
 
 


This is one of our favorite photos of all time.  An Indian gun crew assembles the two piece gun tube in the 1880s.  In our view, that photo makes history come alive. 





The French 75 artillery shell M1897 with driving band.






From the excellent website,    http://riv.co.nz/rnza/hist/shrap/fig6.htm   we get this image of an 18 Pdr. Shrapnel shell of 1904.  Note the driving band.


 
 


 
     However the big question is, would misalignment of the separate sections of rifling severely impact accuracy?  Would not the widened groove cut into the driving band cause instability and thus, inaccuracy?  However, Mike and I do not believe that the large collar, covering the junction of the tube sections has anything to do with allowing an incremental misalignment.  We believe it is simply a tightening device, probably operating via helical camming surfaces.  We believe it’s purpose is to draw the two tube sections together tightly without any rotation on their part whatsoever.  Are we all wet here or what?  We still have not read anything which specifically explains these markings.

     Anybody else have a theory?  Nobody will jump on your back if it sounds a bit strange.  Remember, we are definitely in “Strange Territory” here.  A line from Kiplings poem, Screw-Guns , proves that:  “For we fancies ourselves at two thousand, we guns that are built in two bits.”

Mike and Tracy
 
 
 
 
 


 
 
 
 

[The Jeff]

Here's a picture showing some more alignment marks on a 10 Pdr. Screw Gun: http://www.ima-usa.com/media/catalog/product/cache/1/small_image/b6cef2188d4eb103760549d3dce283bd/s/c/screw_gun-17.jpg


I'm not exactly sure if that will tell you anything, but possibly it could shed some light on the issue?

[Cat Whisperer]

Perhaps, the alignment marks were to indicate a range - put it somewhere in here - check visually the alignment by looking through the tube - AND if the alignment is not within the two index marks, then the wear/stretching of the tube is past the point where it is safe to fire.

Perhaps is the key word.

[seacoastartillery]

      Thanks Jeff, yes they do indicate an alignment, but in this case, an alignment of parts that make up the Breech Block Assy.  Your close-up photo is included within the first one displayed below which shows you how those parts fit together to make up the Breech Block Assy.  This is a British gun with uses the French DeBange Breech System which features a Mushroom head and Spindle Assy with a threaded spindle end which screws into the back of the Breech Block to secure the Obturator which is composed of a cover of canvas which encloses a pad made of Asbestos and Tallow.  Upon firing the piece the hot pressurized gasses push equally in all directions to drive the projectile down the bore and drive the Mushroom Head backwards which squeezes the greased asbestos pad outward against the tube just aft of the Chamber to keep those hot gasses from escaping.  A strong coil spring is also compressed, which  then returns the head forward as gas pressure dissipates.





Here you can see a close-up of the markings which we have read about when we studied this DeBange Breech Block 2 or 3 years ago.  As we understand them, they all line up when the proper sized spacer indicated by the smallest arrow is installed.  The thickness of the proper spacer is dependent upon the loosely, thickness toleranced asbestos and lard pad.  When the asbestos pad is correctly placed and the spacer is the right size, then the Mushroom Head is tightened with a spanner or clamping wrench until all the arrows line up.  At this time the pad should be just below flush with the chamber wall of the tube.




       Cat Whisperer, we don’t understand what is stretching and is it a radial or axial or twisting (torsional) type of stretching.  If you could expand on this we would be grateful.  Thanks.
Tracy and Mike

[Cat Whisperer]

The key word was 'perhaps' - because from the picture it's difficult to tell which pieces are which, hence an unknown as to where the stretching may/may-not occur.

But, a range is indicated as being OK.  Hence my guess as an indication of wear/stretching.

[shred]

I'm thinking the screw threads are on one half of the barrel and the other has a giant 'nut' on a flange; sort of like a gas flare fitting-- with alignment features, the tubes always go together in the same relative orientation, then turning the nut tightens them together.  The wear would be how far you have to turn the nut to pull the halves together.

Edit-- this drawing appears likewise to show a combined trunnion 'nut' and flange on the breech end of the muzzle section, though the IMA pictures appear to show a more complex mechanism to draw the halves together that's ahead of the trunnions for the BL model (possibly a worm-drive or right-and-left-hand threads on either end?)

 

From: http://riv.co.nz/rnza/hist/gun/rifled10.htm

One more edit: Found what might be the original source document for the above pictures, and contains a description.  I think this is the answer, although the BL model obviously differs some in construction.

http://books.google.com/books?id=19agAAAAMAAJ&dq=Elswick%20Ordnance%20Mesurier&pg=PA175#v=onepage&q=Elswick%20Ordnance%20Mesurier&f=false

The gun consists of two parts, the breech and the muzzle portions each weighing about 200 lb., which is considered a maximum load for a mule's burden. The breech portion is arranged to contain the entire charge; it is formed out of a solid block of steel, the front end being furnished with a screw-thread to receive the junction nut. The muzzle portion consists of the steel chase and a wrought-iron ring which is provided with trunnions. This ring in future will be made of steel. The trunnion piece cannot be removed from the chase on account of a sight ring, which is permanently attached in a position to allow sufficient play for this the junction nut. The tw.o parts of the gun are united by means of this trunnion ring in conjunction with a spigot and faucet joint. A steel gas ring is also introduced in the joint to prevent any escape. A key or feather on the muzzle portion fits into a recess in the breech portion, to ensure the two parts coming together correctly. When placed in position the union is effected by turning the junction nut in the direction which is indicated by an arrow and the word " Tighten " on the right trunnion until a line on the latter accords with a similar line on the top of the breech; a few moderate blows with a hammer will suffice to ensure this, but a trunnion-block must be used to protect the trunnion itself from injury when receiving the blows.

That linked book contains a trove of useful drawings and information on markings, construction, stats and sights of British period guns.

[Cat Whisperer]

So the alignment is indexed (good idea) and the wear would be stretching/wear of the threaded device holding it together, no?

[little seacoast]

Also explains what that Sikh was doing standing there with that bloody great hammer in M&T's photo.

[shred]

So the alignment is indexed (good idea) and the wear would be stretching/wear of the threaded device holding it together, no?

That and the threads on the breech part is what I think--  Screwing and unscrewing threads in a hostile environment is likely to get abrasive particles wearing them down over time.  There may be some gas leakage as well, even with the washer, so sooner or later the threads will get worn enough to weaken the joint.  It would be bad quite distressing if the front half of your Screw Gun decided to proceed downrange with the shell inside.   

[Cannoneer]

That book is a good find, shred.

"A steel gas ring is also introduced in the joint to prevent any escape."

Perhaps the authors use of "steel gas ring" is a simplification, actually meant to describe some form of compression ring. This might explain the incised 'wear' marks, that were intended to inform the gunners when the ring needed replacing.

[seacoastartillery]

    Going back to what we wrote on the 29th,

    "However, the big question is, would misalignment of the separate sections of rifling severely impact accuracy?  Would not the widened groove cut into the driving band cause instability and thus, inaccuracy?  However, Mike and I do not believe that the large collar, covering the junction of the tube sections has anything to do with allowing an incremental misalignment.  We believe it is simply a tightening device, probably operating via helical camming surfaces.  We believe it’s purpose is to draw the two tube sections together tightly without any rotation on their part whatsoever.  Are we all wet here or what?  We still have not read anything which specifically explains these markings."

Taken from what Shred quoted, it seems quite clear that no misalignment of rifling ever occurred despite wear of some components.


    "The gun consists of two parts, the breech and the muzzle portions each weighing about 200 lb., which is considered a maximum load for a mule's burden. The breech portion is arranged to contain the entire charge; it is formed out of a solid block of steel, the front end being furnished with a screw-thread to receive the junction nut. The muzzle portion consists of the steel chase and a wrought-iron ring which is provided with trunnions. This ring in future will be made of steel. The trunnion piece cannot be removed from the chase on account of a sight ring, which is permanently attached in a position to allow sufficient play for this the junction nut. The tw.o parts of the gun are united by means of this trunnion ring in conjunction with a spigot and faucet joint. A steel gas ring is also introduced in the joint to prevent any escape. A key or feather on the muzzle portion fits into a recess in the breech portion, to ensure the two parts coming together correctly. When placed in position the union is effected by turning the junction nut in the direction which is indicated by an arrow and the word " Tighten " on the right trunnion until a line on the latter accords with a similar line on the top of the breech; a few moderate blows with a hammer will suffice to ensure this, but a trunnion-block must be used to protect the trunnion itself from injury when receiving the blows."


     Although this refers to the older 7 Pdr. ML design, we think it would be ludicrous to believe that the 'Key' feature would be deleted from the newer 10 Pdr. BL design.  Misaligned rifling was always a non-starter for us.  Back to the meaning of the markerings, we think that Shred said it best yesterday when he wrote:

"I'm thinking the screw threads are on one half of the barrel and the other has a giant 'nut' on a flange; sort of like a gas flare fitting-- with alignment features, the tubes always go together in the same relative orientation, then turning the nut tightens them together.

The wear would be how far you have to turn the nut to pull the halves together. "

CW said it all in very few words:

                 So the alignment is indexed (good idea) and the wear would be stretching/wear of the threaded device holding it together, no?

     Yes, and maybe a bit of wear on tube threads as well.   

     The "LIMIT FOR WEAR"  markings on the large tightening collar or nut almost certainly refers to 'wear' on the threads of the tube section or sections and on the threads of the tightening collar or nut, itself.  The message is very clear to us:  "If you tighten this assembly beyond these limits for wear, you are risking gas leakage or complete, catastrophic, failure of this critical joint.

Mike and Tracy