13-pounder AA gun

[Great War Truck]

Could someone please explain the physical differences between the Mark 3 QF 13-pounder AA gun and the 13-pounder 9-cwt gun. I understand that the latter is actually an 18 pounder with a liner to allow it to take the smaller round, but visually what are the differences between the guns? Thanks. Tim

[centurion]

The 13 pounder Mk 3 had an additional spring in a casing above the standard recoil mechanism to return the gun to its firing position when pointed up at an acute angle. The 13 pounder Mk 4 also had this but a shorter barrel. On the 13 pounder 9cwt changes to the recoil system appear to have been contained within the original casing. This gun did indeed fire a 13 pounder projectile but using an 18 pounder charge contained in a necked cartridge so the breech would have been chunkier than on an 13 pounder Mk 3. The 13 pounder Mk 4 fired a French 75mm round and had a French style breech.

[Rockdoc]

Centurion's not quite right in this case because the twin-buffer arrangement was standard only on the Mk 1 high-angle mount with the 6-cwt Mk III gun and on the unrelated Mk IV gun, only six of which were used and they were supplied complete by Elswick in 1915. The Mk II HA mount for the Mk III 6-cwt gun only had a single buffer, as standard at least. The 9-cwt was a sleeved-down 18-pdr that retained the breech so that, as Centurion said, the larger charge would give a higher muzzle velocity. I believe that the rifling scroll was changed so the rotational speed was retained. Using the normal scroll pattern is said to have led to problems with the igniferous fuzes used at the time. The Mk III HA mount was a Mk II modified to take the bigger piece but proved inadequate and was quickly replaced by a beefed-up version, the Mk IV.

The single buffer arrangement was not always considered adequate and 73rd AAS at Salonika made some modifications, with the IOM Workshops at Salonika, to swap the 13-pdr buffer for one from an 18-pdr, for example, and I've found at least one example where a second buffer was added to a single-buffer gun. This caused all kinds of problems with the balance of the whole thing and the experiment was short-lived.

The easiest way to tell the 6-cwt and 9-cwt guns apart is by the relative length of the buffer and barrel. On the 6-cwt they are similar but the 9-cwt barrel is much longer than its buffer. The 6-cwt Mk III and Mk IV can be most easily distinguished by the mounting position of the buffer piston-rod on the breech casting. The Mk III has the interrupted-screw type and the piston-rod is connected at the rear of the casting. The Mk IV has a Nordenfelt breech, like the French 75mm, and its operating handle sits over the breech casting so, to make room, the piston-rod connection is adjacent to the end of the barrel.

From Wiki

13-pdr 6-cwt Mk III

13-pdr 6-cwt Mk IV

13-pdr 9-cwt

Keith

[centurion]

Centurion's not quite right in this case because the twin-buffer arrangement was standard only on the Mk 1 high-angle mount with the 6-cwt Mk III gun and on the unrelated Mk IV gun, only six of which were used and they were supplied complete by Elswick in 1915. The Mk II HA mount for the Mk III 6-cwt gun only had a single buffer, as standard at least.

According to Hogg the mark 2 mount also had the additional spring but contained within a common casing with the rest of the recoil mechanism. This was I believe sightly larger that the recoil casing on the ordinary 13 pounder so as to make room.

All six lorry mounted Mk IVs eventually saw service in France along side the Mk 3

[Rockdoc]

They did, Centurion, but there is no known info about where they served. My Grandfather's unit had a pair of Mk IVs and that is the only known position, I was told.

Springs were a constant problem. They were also a problem, of course, on the equivalent field guns but weak springs on an HA mount meant that even more manual labour was needed to bring the gun back to the firing position when rapid fire was of the essence. The Salonika Diaries constantly speak of guns being taken out of action for attention to springs, seals and so on. The springs were fitted as 'nests' so i must have been no end of fun getting them into place at the position and extricating a broken set could have been even more so.

Keith

[Great War Truck]

Thanks for your comprehensive answers Guys. I think it is all coming together now. So the Thorny at Duxford has a 9-cwt gun on it? Thanks.

[Great War Truck]

So. To continue with my learning process, what sort is this one? Thanks

[Rockdoc]

Yes, the Duxford Thorneycroft has a 9-cwt gun on the back. That's one in your photo, too.

The platform on an AA lorry is lower than when the same chassis is used for other purposes. The base-plate of the HA mount is bolted directly to the chassis members and the floor is made to match. AA Lorries always have cut-outs in the platform for the rear tyres and mudguards were fitted on the upper surface. The Duxford version has the low-height version that I believe were used as a safety feature when the lorry was at the ready. It is far too low to be of the slightest use when travelling and I've seen photos of deeper ones. The deep ones would have been a serious trip hazard and must have been unfastened to be replaced with the other type. This set-up was not universal and many photos of guns in action do not show any kind of mudguard. I guess they found it no worse to feel for the gap than to stub your toe.

Keith

[Great War Truck]

Thanks Keith. I was looking at the mudguards on the Duxford J and noticed they were much lower than all of the ones which I have seen in the contemporary photos. Is it possible that when that J was rebuilt the originals were missing and the restorer recreated what he thought was correct. Have you seen anything like that anywhere else? Most photos of 9-cwt guns which I have seen have a small "box" at the front of the buffer. Is this an oil reservoir for the buffer or something am I on the wrong track altogether? I notice the one in the photograph above does not have it. Any thoughts why this might be? Tim

[Rockdoc]

I have seen those before on contemporary photos but, as I said earlier, there is no way that they could have been used during travel. I believe they had a set for travelling and these were swapped over to when the gun was at its position. They take away the gap in the platform but would still be a trip hazard, in my book, so whether they were used in practice I don't know. The Mk IVs used by my Grandfather's Section did not have anything like them so you could do without them.

You see that box on buffers for the 18-pdr as well. The design of buffer was changed during the war and ISTM that the box is only found on the later design.

Keith

[Great War Truck]

There is a more comprehensive clip of film of the Thorny AA gun trucks in action on the IWM site here: http://www.iwm.org.uk/collections/item/object/1060022938

[Rockdoc]

That film was taken around Armentieres and there is a set of still photos that go with it. I believe the 9-cwt guns that form the majority of the content are some of the first to get to France and the film and photos may have been taken for publicity or record purposes something like mid-1916. The second battery, as it's described, has a pair of 6-cwt guns, one on a Mk1 mount, with a separate buffer and spring-case and the second on a Mk2 mount with the combined buffer/spring-case. It's interesting to compare the two AA Sections. The 9-cwt guns always have their stabilisers set and, in the early part of the film, you can see one of the two ASC men - the gun detachment's No 11 - checking the screws are tight before firing commences. By comparison, the other AA Section appears to have been taken by surprise. neither lorry has the stabilisers deployed and you have to wonder at the state of the suspension springs after you see the deflection of the platform every time a shell is expended.

One of the indicator boards we were discussing in another thread recently is shown in use and I think our guesses turn out to have been pretty close.

As to the mudguards, at around 50 seconds, one of the lorries prepares to move a short way. At about 1.00, one of the men lobs one of the deeper mudguards used during normal driving into the back of the lorry.

The whole thing is quite clearly a fake, even if the guns were shooting at real targets. Given the size of cameras of the period it's hard to imagine that a couple of photographers just happened to be running between two Sections while they were both firing at the same plane. The clinchr, for me, is in the last few seconds of the film. The men fire a couple of shells and then start waving their caps about, when it took 20-30 seconds for a shell to reach a plane, on average. The Fokker is shown coming down vertically and is shown nose down and on fire, without much damage if it had impacted the ground at speed. The last frames seem to confirm the trickery as the burned-out remains are shown on the undercarriage with e plane horizontal.

Keith

[RobL]

The clincher for me that it's fake (as well as thinking that the cameraman would have been waiting a couple of hundred yards down the road just in case the lorries decided to drive down the road), is that all the men are wearing their bandolier equipment - photos that certainly look more 'real' of gunners of any artillery piece very rarely show them wearing any form of webbing equipment