Boys .55 Anti-Tank Cartridge



I need some information on this Boys .55 Anti-Tank cartridge. Headstamp K 39 W.I. Was it mfg (K) in England? Is 39 the year? What does the W.I. stand for. I understand its an AP round, unknown bullet weight.

Is this a fairly common round? Any assistance would be greatly appreciated.




Yes, K=Kynoch, 39=1939, W=the code for A.P. and I=Mark I the green primer annulus also indicated AP.



K = Kynoch 39 = 1939; w = Armour Piercing; I = Mark I.

Bullet is 930 gr.

Official designation is;

Cartridge, SA, Armour-Piercing. .55 inch, Mark I

As for availability, this is probabily the most available of all the .55 Boys.


Don’t leave me hanging. Mine is MK II. What is that?



Ray-Major differance between Mk.I and Mk. II is bullet weight. Mk. II is 735 gr. The Mk.1 bullet is 58.56mm long while Mk. II is 50.0mm.


I have a fired case stamped: “K4 1942 WII”. This round was also made in practise (aluminium cored), tracer and drill / dummy wersions. However these are alot less common than AP mark II.


Not counting experimental rounds, there were thirteen different loadings of the .55 Boys approved for service.

They are:

Ball, Mark 1
Ball, Mark 2
Ball, Mark 3
Practice P Mark 1
Practice P Mark 2
Armour-Piercing W Mark 1
Armour-Piercing W Mark 2
Tracer G Mark 1
Proof Q Mark 1
Proof Q Mark 2
Proof Q Mark 3z
Drill D Mark 1
Dummy U Mark 1


The photograph below shows a .55 Boys Mark II AP core and a damaged Mark I AP bullet. This latter bullet was removed from a dug up cartridge headstamped K 39 W I and only the part of the jacket which was protected by the case neck has survived. The visible crimp groove is the lower one for neck stabs, The jacket disappears at the upper, mouth, crimp groove. Remnants of the lead sleeve are visible. The longer, heavier, Mark I bullet is evident.



Here are a couple of those experimentals.

Some work was done on an AP Mark III round which was to have a cone pointed composite rigid bullet. It never saw service but was sealed “for record”. The design was to Drawing DD/L/14087. I do not have one, but do possess what appears to be a ballistic vehicle for the round or some kind of mock-up.

The projectile is blacked turned brass and is loaded into an RG case, although it was probably loaded in the lab at Enfield.

The second is the rather more often seem .55/.303 training round that is mentioned in the Boys manual.



I’ve here not a real cartridge but a .55/22 inch steel adaptor, it’s not the same as discribed in Peter labett book “British small arms ammunition 1864-1938”. It looks genuine, marks on the head “a broadarrow in a circle, HWC in an oval”. It’s in fact a rifled barrel wich meets the measure of a boy’s cartridge. Is there someone who has some information on this?


I have the same adaptor. I believe it is Canadian but I do not know the identity of the maker.



Did the bolt have to be changed to one with an offest firing pin to fire .22 Rimfire from the Centrefire Boys Rifle?


I think if you look closely at the “broad arrow in a circle” that you will find it is a “C” not a circle. These are definntly Canadian. There is no official record that I am aware of that these adapters were ever issued to the troops.

I do not know who “HWC” is.


No. The .22 round is enclosed in a chamber within the adaptor and the “Primer” of the adaptor has a rim fire firing pin. The pin of the rifle hits the “primer” which in turn strikes the .22 rimfire. I hope that explanation is comprehendable!



@TonyE - Thanks - that explanation is comprehendable. Does something have to be screwed in to the base of the adaptor after the round is inserted. I guess the fired case would have to be pushed out from the front with a wooden rod or similar. Is this correct?


Spot on Falcon! I will dig mine out and post pictures.



Thanks, the pictures should be interesting. What is the legal status of these adaprors in the UK? Is it classed as a firearm as it has a firing pin, chamber and barrel? It also has a barrel shorter than 12 inches. Isn’t 12 inches the minimum legal barrel length in the UK.


On a more mundane side of the Boys cartridge… The picture shows the “powder” from a pulled Boys cartridge. Is this a typical type of powder for large military rounds? As a reloader I’m not familar with it.




Falcon - Ssssh!

Jones Yes, that is Cordite, named for obvious reasons. It was used by the British for everythig from small arms cartridges to big Naval guns.

In addition to the cord type, it came in chopped and flaked varieties.
That in the picture is Cordite MDT 7-2. MDT means modified tubular and the 7-2 is the outside and inside diameters of the tube in hundredths of an inch.



That is British “Cordite” powder. It is made from a combination of Nitroglycerine and Nitrocellulose. It is usually found in that spaghetti-like form.