40mm Bofors Clearing Blanks

[ron3350]

I show two Bofors Clearing Blanks. What is the difference between the Mk 1 to Mk 6 Blanks ?
This is a Mk 3 Australian case by MD (Derwent) re-used by MF as a Mk3 Clearing Blank. If 1949 the 1 is crooked.
Cut-off case shows the case shoulder. RW 49 = ??. Case length is 280mm.

The second example is a Mk6 by RLB 1952 case but 9/55 on markings.8 crimps fired with protective cover and cardboard case protector.
Case length is 254mm.
I realize the case must clear the jammed projectile base and tracer but is the length different with various marks? Not much information.

 

[ron3350]

I only meant to send 1 set of photos but had a problem and 2 sets printed. My error. I await all replies. Ron.

 

[AE501]

Thank you for showing Clearing cartridges which do not have a crimped mouth like an oversized small ams blank.
Blank and Clearing cartridges are two different things.
As you know, Blank are mainly used for getting raw recruits used to the sound of gunfire and for saluting purposes. Because there is no projectile to push, a more vigorous propellant is used, previously gunpowder but now a fine grain chopped multi tubular propellant is favoured.

Clearing cartridges are something completely different. The propelling charge for every round/shell with a copper driving band has a small amount of lead or tin foil included in with the propellant, which melts and forms a brittle amalgam with the copper deposit left in the bore. This is pushed out by the next projectile which will leave its own brittle deposit.
Over time there may be a build up of copper and a Clearing cartridge, with a much larger amount of lead/tin foil, is used to make a better de-coppering job. Clearing cartridges are mainly used by gun workshops, who want to clear the bore of as much debris as possible before examinations. At one time bore impressions were taken with hot water softened gutta percha, but I expect they now have other methods.
You do not get lead/tin foil in most tank/anti tank APDS and APFSDS rounds because there is no copper, only nylon and aluminium deposits.

 

[ron3350]

I assumed a clearing blank was to fire an obstructed projectile from the bore when a gun was otherwise out of action due to the case separating from the case when extracted?? and not fired. That was why the case was shorter to clear the projectile. I disagree with your clearing copper etc from the bore explanation. Am I wrong??

 

[Quatermass]

A previous thread on the topic which may be of interest to you Ron.

http://www.bocn.co.uk/vbforum/threads/94903-Clearing-cartridge?

I have a Bofors case with the same dimensions as your first case and mine is stencilled on the side expulsion and a second Bofors case that resembles a crimped small arms blanked stencilled clearing.

 

[ron3350]

Thank you all for the input. I read all the old postings and now understand more about de-coppering and clearing. I have the 20mm Oerlikon clearing blank.
The 40mm cases are hard to find. Still no answer to the differences between Marks 1 to 6, case lengths or charges.

 

[radioman1]

Hi Have a question about the RLB case.

1 Is it the case without the filler markings?

2 What I don't understand is the primer Its a MK 12 N7 Filled at MY 7 85 lot 52

3 I don't know that filling station could it be not from the UK

4 Having worked on many thousands of Bofors not come across that filler or maker of primer

Hope this helps

Steve EX RNAD Trecwn

 

[AE501]

​This was a 40mm Mk IV Cartridge case made by Manchester Die Castings Co. UK in 1943 to empty design RW 49.It looks as though it was converted to a Mk 3 Clearing cartridge in 1949 in Australia.


The next case is a 40mm Mk 4 cartridge case made by Royal Laboratory Birtley UK, for RN.
‘Clearing' can be read on the top line of the cartridge, despite markings damage and on the next line down I can just
make out ‘RNP' which would probably have been the original Royal Naval Propellant Factory which made the propellant and filled the cartridge.
The cartridge has undergone a transition since then to naval Mk 6 clearing charge. Perhaps 1982 is a lot number, because work on the filling was done by RN station BP (Bull Point) in 1955.
I would say this has probably been made up as a gun clearing charge aboard ship, to a tried and tested design.


The primer manufacturer could well be from an as yet unknown maker. This is quite a late date and may not yet be known to BOCN members yet.


I can only help you as far as Mk 4 empty cases.

Mk 1 Would only take a No 18 primer
Mk 1* Converted to take the No 12 primer by enlarging the primer hole to take an adapter for it.
Mk 2 As Mk 1* but new manufacture.
Mk 3 Canadian design. Similar the Mk 1 but primer hole made large enough to take a No 12 primer without an adapter.
Mk 4 UK manufacture to the Canadian design.
I would think that if there was empty case Mk 5 and 6 they would also take a No 12 primer.

It is possible to read this by highlighting it as though you are going to copy or cut it

 

[ron3350]

I did not take any notice of the primer. It is the normal primer used in Bofors till the end around 1992 made by MF factory and filled by MY at St.Marys in Australia. It would be a replacement and I should find a British one to match the date.

 

[radioman1]

Info on propellant charge


1 Propellant marked as RNP was manufactured at the Royal Naval Propellant Factory Caerent from 1939 to 1965


2 The filling of the cartridge whould not have been done at Caerwent


3 I don't think that a RN warship would be able or allowed to make up the cartridge as it has to be done at a Armament Depot

4 The normal fitted UK
primer would have been a MK12 N3 or a MK12 N4

 

[AE501]

I went round RNPF Caerwent in 1956 and I take your point that it would not have been filled there. More likely RNAD Ernesettle or some other RNAD. There were more of them about in those days.
There is an element of RNAD rework rather than ROF about the clearing charge, as though it was for getting a stuck fast shell out of a gun rather than de-coppering. Perhaps it was produced by a TIALD.
The primer in his cartridge is a No 12 Mk N7 but it is a much later date.

 

[ron3350]

Ignore the primer in the RLB case. I remember 5 years ago I cleaned the threads using this primer and it was not original to this case. Sorry.
To AE501 I cannot accept the MD as Manchester Die Castings . MD Bofors (and 20mm OeZ) were made by Derwent Park Ammunition Factory,Glenorchy,Hobart,Tasmania. Bofors in 1943 and 1944. Oerlikon in 1943-45. All these cases were sent to mainland Australia for loading. Derwent made 690,813 40mm cases and the works were closed in March 1945 owing to a surplus of these particular munitions. Both calibres with MD headstamps are relatively unknown so I can see why you would think it was a British case. Why would there be two factories using the same MD marking?
MD also made 2" Mortar bodies and tail fins. The RLB case has an extra line of print not shown. It is ALN-001.

 

[ron3350]

I was a bit hasty with the same factory code statement. Of course two different companies can use the same code. eg. MF and MG can be Australian of French or Brazil. The fact that MD Bofors cases are found in Australia (and Tasmania) points to Australian surplus and reuse as a clearing blank in my opinion. The facts on MD production came from an ACCA article in Journal 98 of 2004 by J.Green, Tasmania. Same MD letters with a slight cant (not straight) and an asterisk under it.

 

[AE501]

You obviously know a lot more about those Australian factories than I know about Manchester Die Castings.
To put some more perspective on the de-coppering Clearing cartridges.
All the tank and anti tank guns from 2 Pr (except Mk 10B for Littlejohn due to the absence of copper) to 120mm APDS for Chieftain and probably beyond, had a Clearing cartridge as well as a Blank. The amount of propellant in Clearing and Blank cartridges is very similar, but may be a different type, but Clearing cartridges have twice as much lead/tin foil as the normal cartridge. 40mm by virtue of having to defy gravity and reach the highest possible ceiling also had the same and because it had a high rate of automatic fire, with high barrel temperatures, severe coppering was found. This is why 40mm was the most prevalent Clearing cartridge.
A shell stuck in the bore of a gun is a very rare event and you would have to remove it under local arrangements by either mechanical means or if the gun was urgently required, by explosive means. In the latter case it would be a good idea to find a few Blanks and add Clearing to the description so that they did not get shot off and you would have something to help to get a shell out, by removing some of the propellant relative to how far up the bore it was.
Out of interest, the 2 Pr Clearing cartridge used a shortened standard cartridge case.