3.7” 6cwt Mountain Howitzer Mk 2-3
3.7” Mountain Howitzer No.2 Mk 1 & 2
94mm Howitzer M1A1
(Current CAINUK designation Howitzer 94mm L2M2 or L2M3)
Construction: Multi-tube dismountable
Breach: Manual falling block,
Calibre: 3.7” (94mm)
Length: 53.74” (1365mm) without Muzzle Brake
Weight: 5cwt 3qrts
Type: Split Trail, Dismountable
Elevation: +45 to -3
Top Traverse: 20 L, 20 R
Recoil: Hydro-pneumatic (variable)
Weight in Action:
Propellant: Semi-fixed, brass cased
Charges: 6 + Supercharge
Projectile: HE (19.5lb, 28lb, 32lb), Smoke (22lb and 29lb), Illum (29lb), AP (HEAT), Carrier (29 and 32lb various Mk.s)
World at war or not, the North West Frontier of India, for all the roads Delhi build, has never changed much, neither have the demands on its garrison. Their standard mountain gun for over 50 years was the 1907 (designed) vintage ‘Ordnance QF 3.7” Mountain Howitzer Mk 1’ (on Carriage Mountain Howitzer Mk 1). Production of this weapon was due to be transferred to India in 1941 before the Coup of 1940 overturned all such plans and isolated India from further supply. By the late 40’s the guns in Indian hands had been refurbished more often than Washington’s axe and were starting to show their age. The Chipanese over the hill might have had something to do with the call to replace them as well.
Australia had undertaken the production of 3.7” ammunition for India in addition to supplying the bulk of her modern Artillery post Coup, and was known to have experimented with very light weight variant on the standard 25pdr field gun. So in addition to investigating other potential sources of supply in Europe and America, Australia was approached to see if the light weight 25pdr was available and if it might fit their requirements. Those specifications were a rather lose, in essence all they wanted was a gun as good as the old 3.7” and if it were a little lighter, or could fire a little further, then so much the better. But it had to break down in to roughly 200lb loads, be tough enough to roll down a mountainside and come together in less than 5 minutes (they were being kind with that one, scratch time for the 3.7” Mk. 1 from saddled to loading the first round was ~ 70 seconds flat).
Unfortunately the experimental 25pdr had not been a success and in discussions between Indian and Australian ordnance authorities it was agreed that this weapon was not suitable for India, but an offer to try something along the lines of making 25pdr barrel for the Mk.1 carriage was floated. The wider search was no more successful, while a number of mountain guns were available all, were 75mm or lighter weapons, with a shell weight around half that of the 3.7”. The only potential candidate being an Italian design in 105mm, then only just entering the prototype stage of its leisurely development. While impressed by what was to become the Otto-Melara Modelo 58, the Indian Army were less than happy with several features, the calibre was an unusual one in Cw service, the weapon was heavier than desirable, as was the ammunition, a vital consideration in mountain warfare.
A new Pack Howitzer was not particularly high on Australia’s list of priorities in 1949, although such a weapon was recognised as something useful. But an air portable howitzer was another matter altogether, the original light weight 25pdr had not been an idle fancy. On this basis Canberra and Delhi reached an in principal understanding, Australia would attempt to make a suitable weapon if India undertook to buy 200 pieces at a 10% premium before anyone spoke of licence manufacture in India. Should the Australian program fail, India would buy 200 additional 25pdrs to help cover some of the cost.
All this left the Indian Army with a clear choice, they purchased a number of American 75mm M1A1 howitzers from surplus Russian stocks as an interim measure, and sat back to see what the Italians and Australians would come up with.
The ‘bitza’ project in Australia was given to the Government Ordnance Factory at Maribyrnong in Victoria along with a 3.7” Mk1 inherited from the New Zealand Army and they proceeded with some speed on what was at first seen as a simple task. A new barrel in 87.6mm to fire the 25-pr round was a matter of weeks, and was mounting it in place of the original 3.7” tube wasn’t much harder, however calculations showed that with charges to get a ten thousand yard range, the old 3.7” recoil system would be in trouble. A proof shot at the Port Wakefield range to test this, confirmed the engineer’s figures and saw the breech of the trial weapon buried 2 feet into the South Australian dirt. Worse the muzzle blast was ‘objectionable,’ which is one way to describe a 10 foot ball of flame and fury. A muzzle brake was put high on the list of priorities back at Maribyrnong, along with adapting the 25pdr’s top carriage to fit in the old 3.7” lower.
Despite the Austral-Indian political falling out in late 1950 over Aceh and Australia’s switch to an Australian First defence policy the Pack Howitzer project lost no steam. If anything Australian demand outgrew the Indian need. Pinned to a continental defence, and faced with vast distances and poor infrastructure, the Australian Army had little choice but to embrace air-transportability for its first wave of reinforcements to nay hot spot and those troops would need arty.
At this time Maribyrnong’s principal effort was into very high altitude anti-aircraft guns mostly in 3.7” calibre, one part of this being the design of low drag shells. 3.7” had also been accepted in principal by the post war Commonwealth Joint Committee on Artillery as the new field gun calibre to replace the 25pdr. In light of all this it was reasoned that a change back to 3.7” could only help the Pack Howitzer program, the commonality of calibre being a useful benefit, but on a technical level a more ballistically efficient projectile demanded less from the gun firing it to reach a given range. It also allowed the designers to stop messing about trying to cobble up a ‘parts-bin’ gun and just do the job properly, the cost was a blow out in time and budget.
It took almost five years to shake through all the issues and come up with solutions, eventually a muzzle brake of about 35% efficiency was fitted to an18 calibre barrel with a brand new recoil system (copied from German practice) all mounted on a new carriage derived from the original Mountain Howitzer Mk 1 pattern but actually a complete redesign.
The Ordnance QF 3.7” 6cwt Mountain Howitzer (Aust) Mk 2 barrel comes in two parts (A-tube and B-tube) with a detachable breach and muzzle brake. It comes apart into 5 separate elements for pack transport, the two barrel components, the Breach Ring, the Breach Block and the Muzzle Brake. The A-tube has a felt lined transport sleeve, the B tube has a special tompion that includes a push-through cleaning plug and the Breach Ring is the second heaviest part of the whole artillery piece.
The two-part barrel goes together in a rather unusual way, in that the two components are concentric. The A tube is essentially the liner (bore tube) and fits inside the B tube. This eliminated the weakness of a normal joint as with the old Mk.1 and also allows the gun to be re barrelled by simply replacing the A tube. The B tube screws into the Breach Ring with the help of a worm screw fixed into the Breach Ring, when the B tube is in place, the A tube (coated with the thinnest smear of copper grease) is inserted into the B tube through the Breach Ring. A flange on the A tube is threaded to screw into the Breach Ring for the last inch or so of movement; the special spanner for this is also used to turn the worm screw for the B tube, tighten the recoil nuts and torque up the cap squares. The last part of the assembly is the Breach Block itself, this just drops into the Breech Ring, a pin connected it to the operating handle linkage, but it serves as a final lock to hold the whole thing together, the Breech Block will not fit unless everything important is screwed up tight and while it is there, nothing can come apart (the Muzzle Brake just screws on to the muzzle end of the A tube with a cotter to lock it).
Getting it all to pieces apart again is the fun bit. Firing the gun not only expands the A tube, but the rifling and the B tube thread both twist to the right while the B tube has a LH thread but comes in from the other direction, thus the whole thing self tightens when the shell tries to turn the A tube clockwise.
Separating the two tubes comes down to the worm gear. As the B tube has an LH thread into the Breech Ring, when it is rotated anti-clockwise (seen from the breech) to loosen it, the friction between the tubes turns the A tube anti-clockwise too. But the A tube has an RH thread into the Breech Ring so it is also loosened off, then because the B tube is moved forward and the A tube backwards by their respective threads there is a second camming action to break the two parts free of each other. However you have to get them turning, and having been locked up tight by a few rounds, the friction holding the whole lot together is terrific, so it’s the worm screw that provides enough leverage to overcome the static friction and get the B tube to turn.
It is a bloody clever (@ note if I do say so my self) adaptation of the method used in the 3.7” Mk 1. In effect the ‘nut’ that was used to join the two halves of the Mk.1 barrel has moved back to the breach end and been combined with the Breech Ring,
The falling block Breech mechanism is fairly simple, being based on the 25-pr Mk II type with the operating lever changed from a single handle on the right hand side, to a stirrup arching across the top of the Breech Ring to allow use from either side in cramped situations. Other than that the only real change was the removal of the foot trigger, firing being done by the same hand lever system used as a back up on the 25-pr.
The recoil system is a conventional single tube Hydro Pneumatic set up slung under the barrel. As the ordnance is fairly light the whole thing is more buffer than recuperator, and they under estimated things with the Mk.1 which lacked vigour in returning to battery at high elevations. The working pressure was jacked up in compensation and that could lead to seal problems at high sustained rates of fire in the Mk.2, but that was all sorted in the Mk.3. The tube fits into a deep socket in the Recoil Slide under the barrel and locks with cotter pinched up by a butterfly nut, the recoil rod extending back though the socket running in a Oilite bush to the Breech Ring. Between them the rod and bush locate the barrel in rotation, so the fit is quite important to accuracy and the bush is probably the more replaced part on the gun. Slotted in between the butt end of the cylinder and the bottom of the socket a little leaver that rotates around the rod and controls the valving to adjust the recoil travel (higher elevation = reduced recoil distance), the dangling end working against the fixed cam on the Top Carriage. The actual leaver is held in the Recoil slide by a circlip, its more like a simple spanner really, and the valve end is hexed to fit it. It looks curse, it is crude, and works so it’s not stupid, but making sure the two index together is probably the fiddliest job in assembling the piece and something racing crews usually get around by losing the spanner and leaving the buffer set to full hard.
The split trail Carriage Mountain Howitzer (Aust) Mk 2&3 is a fairly normal gun carriage in most ways, it’s two major eccentricities are the cranked axles (10” offset) to allow better cross levelling on hill sides and to provide some flexibility in height, along with the two part trails. I suppose iron rimmed wooden cart wheels might rate as an eccentricity too, certainly an anachronism, and the Indians are the only people to really use them. But there’s no point lugging pneumatic tyres up mountainsides, it just annoys the mules. For the everyone else someone had a rush of common sense to the head and specified 16” wheels with the standard Jeep/Land Rover bolt pattern. This isn’t just handy if you have a puncture, no, it means 14” Jeep wheels fit too, as do the 18” wheels found on a lot of light trucks and the 1950 Rolls Royce Silver Wraith, a polished set of which does much to lift the tone of any Saluting battery, Bentley hubcaps for racing guns obviously.
Each of the trails is divided about half way between the Lower Carriage and the spades. The four parts are welded box sections and connected together by a pin working through a circular (pie shaped) splined coupling. A cam lever on the pin clamps the splines together to lock the joint in almost any position over a 70-degree arc. This was done because the adjustable feet on the old 3.7” didn’t offer enough support for the more powerful gun, but there was still a need to adjust the hight of the trail legs on cross slopes. The articulated legs offer a greater range of adjustment and by setting each section to its extreme position (inner up, outer down) you can shorten the overall length of the trail by about two feet, which saves shovel time when digging a gun pit. The pivot where each leg meets the body has a similar splined coupling offering a total spread of 45 degrees between the legs, but they can be locked to odd angles if need be. All in all it’s a very flexible arraignment and a blatant rip off of the Otto-Melara Mod 58 the Indians had been eyeing off.
The Lower Carriage (LC) is another welded steel structure, a simple rectangular platform (hogged out for lightness on three sides) with T section reinforcing ribs on the underside connecting the fittings for the trail legs at the rear, the stub axles at the forward corners and the socket for the Top Carriage (TC) pivot. This socket is set in the centre of the front edge and incorporates a hefty locking catch on the Mk 3. The detachable brackets for attaching the shield act as traverse stops and are bolted to the top of the platform.
The Mk 2 used the traverse sector to lock the Top and Bottom carriage together, the sector was raised up on spacer blocks (in cross section it looks like a T headed rail) and there are lugs on the TC that fit under either side. It was a good way of making one thing do three jobs, but it left the greasy bits exposed and they wore quickly, the gun then loosened up and lost accuracy (known as ‘Getting the shakes’). The Mk.3 carriage replaced all that with a ball screw that was both lighter and could be sealed with rubber bellows and put the fixed pivot on the LC rather than the TC. Incidentally the Mk.3 is a good few seconds quicker to mount, and dismount making it the version of choice of Gun Racing, which is why the Mk.2 is the agreed international standard, racing is the best way to ruin a gun in record time and there’s no point to throwing money away.
The Top Carriage is not only the interface between the recoil system and the gun on one hand (the Recoil Slide) and mother earth via the rest of the carriage, but it’s where the traverse and elevation controls do their thing. I’ve already mentioned the traverse gear (there is 20 degrees L and R, 40d total), in comparison the elevating gear was just a simple worm and sector in the Mk 2 (replaced with another ball screw in the Mk 3, arc +45d to -3d). The trunnions are mounted well back against the Breech Ring to provide the maximum elevation for the minimum hight, the barrel heaviness this causes is counteracted by two tension spring equilibrators that run from the front of the top carriage and connect to the back of the Recoil Slide behind the trunnions. In the flesh the TC is just another rectangular weld-meant with the receptacle for the pivot at the front edge and reinforcing ribs running along each long side, rising to the trunnion sockets at the rear. Oh yes and up the middle is a small curved ramp that acts as a cam for the recoil-compensating valve. The cross levelling bracket for the standard dial sight (No.’s 7 though 12 as available) is on the left hand side and comes complete with a 25-pr style gun-rule. The controls are laid out as for the 25pdr too, with the one difference that all control wheels are replicated on both sides of the piece, even though it is single laid (aimed by one man). This is a nice touch for working in confined spaces, but the lack of a decent seat in either position has annoyed generations of gunners.
The Recoil Slide (RS) is the heaviest single component, this but was originally a forging but the Mk.3 shaved off a pound or so by welding it up. It is basically a short sleeve that supports the barrel, with a socket slung underneath for the recoil cylinder, a pair of trunnions sticking out the sides and elevation sector bolted onto it and a lot of odd mounting lugs scattered about. The rod from the recoil cylinder protrudes from the back of the slide so the Breech Ring can be fixed to it (by a nut, 3/4th” Whitworth for the curious).
The gun can be towed by a Land Rover, Jeep or light truck, but don’t expect to get out of low range unless the optional solid un-cranked axles are fitted (even then 30mph on roads is about it). Although there’s also a field modification that involves four bronze washers and a pair of spare equilibrator springs, where by two washers are fitted between the axel splines on each side, the axles are turned upside down and the springs are stretched between the handspike sockets and brackets welded to the trail legs… just don’t get caught. It’s much easier for all concerned to just run it up into the back of the damn truck.
Lastly a word on the shield, it comes in four sections, centre, left and right, with a 12” folding top piece which is one identifying feature between Australian and Indian production, Maribyrnong guns have a wavy top edge, Indian ones are cut square. The shield also has another two optional extra ‘wings’ which extend the shield out another 18” to either side, an idea lifted from the old 3.7” and up on the NWF just as useful in 1987 as it was in 1907 when the locals start sniping. Frankly the shield is a pain in the backside, which is no reflection on the people who designed it, as I defy anyone to make four square yards of ¼” steel plate a convenient thing to carry. While awkward and often superfluous, the shield does add weight to an otherwise ‘lively’ gun and is a valuable addition if any sustained fire is on the program. Failing that the bits add a nice bit of overhead cover to any sanger, slit trench or dugout.
Putting the whole thing together is actually very simple (Mk 3):
-Lower Carriage assembled with wheels and trail, the whole rough levelled
-Top Carriage fitted (usually a 3 man job), traverse gear connected, shield brackets fitted
-Recoil Slide fitted (~ 202lb on its own, with recoil cylinder 255lb), cotter and cap squares bolted up,
-Recoil Slide elevated to maximum and equilibrator springs connected
-This is the tricky bit. Three men grab the recoil cylinder and pull down against the springs, two more men stand on the trail legs to act as ballast and a fifth man juggles around trying to slide the elevating pinion across into mesh with the sector.
-B tube (~190lb so 4 men) is inserted through the Recoil Slide sleeve
-Breech Ring positioned on B tube, start to screw it up while slipping the recoil rod through its hole in the Breech Ring and start the nut
-A tube inserted through Breech Ring into B tube, pushed home and the thread started by hand, centre shield section fitted (or not, the shield being a bit of an optional extra)
-B tube finished, Sight Bracket fitted, A tube starts to be torqued up, left shield fitted Muzzle Brake fitted loosely
-A tube finished, start on recoil rod nuts, fit right shield
-Finish recoil nuts, fit Breach Block tighten the Muzzle Brake, fit the sights, cross level and it’s done.
Honestly it’s a lot easier than it sounds, there are between 6 and 9 men in a detachment depending on establishment, all bolts on the go through keyhole slots and apart from the recoil nuts, every fastener will only need a final nip with a spanner to get it tight. If you don’t believe me, watch the ‘Gun Race’ at the next Royal Tournament or Edinburgh Tattoo.
Time on a parade ground (by the drill book) about two minutes for a good crew, in the field three to five and under fire 105 seconds flat!
The 3.7” pack as been used just about everywhere since it was introduced, India’s North West Frontier Province has probably see more of them than anywhere else, but the 3.7” was the natural gun for paradrop artillery, helicopter delivery and establishing hill top fire bases in rugged terrain.
Authors Note: The Medium Battery I saw out my reserve service (73-75) in was multi-rolled, aside from our 5.5”s we manned a coastal battery (Fort Nelson – Nelson Victoria) and held the 3.7” 6cwt (we even had the 4.2” mortar for a few years), this multi-roleing was (and still is) standard practice for reserve artillery units as if nothing else it’s good training, offers a flexibility of service and it’s cheap storage for valuable equipment that needs regular maintenance just sitting still. So I’ve done a bit of work with the 6cwt, all I can say is that it’s a beast to man pack (we never had mules thank god) and a bastard to tow, but a gem to work with. It’s no field gun, and it can’t maintain the same rate of fire, but within its limits the 6cwt can do anything a gunner might ask with a flexibility of ordinate hight (important for crest clearance in hilly terrain) that is only exceeded by a mortar.
The shells used have changed over the years, from the original 19.5lb HE shell of the Mk 1, to the 28lb standard 3.7” WWII AA gun HE and the later version, nowdays the Extended Range 32lb HE with modified charges to keep the pressure down gives it even more reach. Rumors of a Mk. 4 revised to use the new shells with standard charges have been flying around for a decade, but so far that gun hasn’t seen the light of day.
In addition to HE, the 3.7” 6cwt has fired plenty of other shells, the main one being of course Smoke. The 6cwt was and remains the only CAINUK gun to use a White Phosphorus smoke shell (although that hasn’t stopped the regular FA snaffling some 3.7” WP rounds on certain occasions). Commonwealth Artillery doctrine has always been very fond of smoke,
and before WWII the best smoke screens were found to come from Base Ejecting shells. These chuff out a handful of big smoke candles that burn cool and slow in any colour you might whish (a minor bonus). WP in contrast looks spectacular and has an incendiary effect, but its WP filling gets scattered and burns hot, hot smoke rises and a scattered change burns fast. As a smoker WP can’t compete with BE. The reason the 6cwt uses WP, is because in mountains the smoke candles from a BE round tend to bounce and roll ending up god knows where. WP might spread itself about a bit, but it doesn’t go rolling back down the hill towards friendlies.
Over the years both the old 22lb and standard 29lb smoke shells have been issued, the 29lb is the current round and I doubt anyone has fired a 22lb WP shell since 1960.
Odd as it may sound the 3.7” 6cwt has two anti-tank rounds, not that I’d expect to see many around a gun pit if I were you. The first a H/C (HEAT those of Sam) was declared obsolete in 1970 but it was inherited from the Mk.1 and dated back to the mid 40’s. I can’t remember its exact performance, but rumour has it vanilla HE was more effective. The current round is the same HESH projectile issued to the Field Artillery and while I do know what that round could do circa 1970, it’s still classified…