.303 British "RL 1938 B IV" incendiary

D.Dietz’ notes say that it is a rare incendiary. How rare?

Vlad, unfortunately this is not what I would call a “rare” round. Here in the UK you can buy these for about £3 each, or about $4.75. I have seen them cheaper as well. Do you have any .303 rifles? If you have more of them try shooting one somewhere that allows tracer. They are filled with White Phosphorous which is forced out of a hole in the side of the bullet in flight and will produce a spiral smoke trail.

I have a strong feeling that either TonyE or DocAV will jump all over me when I post this because they are right most of the time and I am not.
I believe this is an anti aircraft round. I will say no more than that to reduce my subsequent humiliation.

No Vince, it was used for anti-aircraft fire, but it was principally an air service store. As Falcon says, it left a smoke trail due to the phosphorus being spun out of the weep hole and igniting on contact with the air. For this reason it was often referred to (particularly by Kynoch) as a smoke tracer.

It came into service in 1928, although the prototypes had first been made in 1918. The WWI Buckingham Mark I was the round nosed incendiary, the B.II was the spitzer version and the B.III was the flat nosed type which was designed to punch a better hole in balloon fabric.

The next stage was an experimental incendiary (which evolved into the B.IV) with a step in the bullet jacket designed to cut a larger hole in the balloon fabric so allowing more hydrogen to be dragged out behind the bullet and ignited by the phosphorus.

The B.IV stayed in service for most of WW2, the last examples being made at RL Woolwich in 1943. However, it had largely been replaced at that date by the B Mark VII which was a particularly effective incendiary.

Have a look at this YouTube footage. The first few seconds show B.IV rounds being fired and leaving their distinctive smoke trail.



Just helping TonyE
Tony, what is “air service store”?

An “air service store” means that it was usually issued and used by the RAF.

Here’s a sectioned projectile. The large part in the center is the lead plug. It has 3 deep grooves that run the length of the plug. These grooves allow the phosphorus to have contact with the air so as to burn and produce the smoke trail. There is a fusible plug in the jacket (in this picture it is on the bottom portion of jacket just above the case neck and below the lead plug). Note also the copper cup which sits between the lead plug and lead base. The copper cup defines the annular air space and I suspect prevents the lead from melting and clogging the vent hole in the jacket.

I had been looking for one of these smoke tracers. Though I don’t collect 303 so I am not well versed in all the varieties, they interested me. Come to find out I had one sitting on the shelf! I remember buying it now only because I thought the stepped bullet looked neat. I had no idea it was a smoke tracer until I came across this thread.


For comparison of the .303” MkIV (left) and MkIVz* incendiary rounds, see below, which had different internal constructions.

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Definitely not rare Vlad, as I have one!

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Guess you have one of these too!!


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I certainly do John! However, mine is one of the VII ball loads. I’ll let you tell the story behind this particular headstamp.

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I also haven’t yet seen anything other than a ball loading in this headstamp. Even these are not common. The headstamp relates to an annexe setup in Hendon next to MJ during WW2 to manufacture the Incendiary BIV (Hence the B in the monogram). Almost as soon as production began, focus switched to making the B MkVII which was less dangerous to manufacture. David Mayne told me only about 9,000 of the completed rounds were manufactured. I would love to see one.

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How did they switch from Mk4 to Mk7 so quickly? What happened to Mk5 and 6?

That is what I was told as well John, although I wasn’t aware of the reduced risks in manufacture. I also wasn’t aware that only 9000 of the ball loaded cases were made. Although, now that I think about it, you may have mentioned this before and I simply forgot.

Vlad - according to Tony Edwards, Mark V wasn’t introduced into service and Mk IV were approved for air service in 1939 but none were made outside of the UK. Tony shows several British made headstamps dated from 1940 - 1943, with unheadstamped rounds also produced.

Tony has a footnote under the MJB 1942 BIV headstamp that John and I posted: “Australia had planned to manufacture the B Mark IV, but instead started production with the B Mark VII. The cases headstamped for the B.IV loads were actually loaded as Ball Mark VII rounds, not incendiary.”
Source: Edwards, A.O. (2011). Headstamp Guide: .303 inch British Service Ammunition. p. 53.

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Maybe I wasn’t clear. Only approximately 9,000 rounds were loaded with the Mk IV bullet. I have no record of the total number of MJB cases produced. However we do know that any surplus cases with the MJB monogram were loaded as Mk VII Ball.

Tony was not correct in his assertion that Australia did not make the B Mk IV.

I queried John Martin about the MJB B Mk IV in a letter to which he replied on 24th August 1971.

He replied that alongside No. 4 factory a small group of buildings were erected for the sole purpose of filling the incendiary bullet. This annexe used the monogram MJB. I understand because of the nature of the filling used in the B 4 cartridge , the cartridge was assembled and completed within the annexe using capped and cordited cases from the main factory, hence the monogram.
The filling used in the B Mk VII made the bullets much safer to handle and these were assembled and completed in the No 4 factory, although I have evidence that one lot was produced in the No. 3 factory (MH).

Hope that clarifies things.


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Thanks John - that is very helpful. I was under the impression that no Mk IV incendiary rounds were actually loaded. I have amended my notes. I don’t suppose you still have the letter do you?

Hi Darren,

Yes, I still have all or most of my correspondence with John Martin. Looking for this reference I also came across the answer to another missing link. He had said the .303 H Mk. IV was eventually made to an Australian specification and could have had H IV (Aust) in the headstamp.

I came across a letter I’d written in June 1971 where I said I’d seen a specimen of this round with the (Aust) in the headstamp when visiting Footscray, so it does exist. I had completely forgotten this. From the description in my letter it’s exactly the same as the more common one with the crimped case with red crimp. Now to obtain a photo of the headstamp.

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Thanks John - I think you need to drop everything else and publish a book!

I had that intention but best I can do with available time is keep trying to do articles. One of my problems is hating to put into print incorrect information. There is too much out there which is completely wrong, but keeps getting quoted as gospel.