British 20mm Aluminium case

I picked this case up at a local gunshow today. The markings on the case wall are “H . E . I . R . 1Z SWN 4P/49” What does this mean?



Looks to me more like H.E.1.R. than H.E.I.R.

According to Peter Labbett’s booklet on the ammo, stamps on the side of the case were introduced in 1946 in order to include more info than was practicable on the headstamp. He lists the normal info provided as follows: the monogram of the filling station; the cartridge type and mark; the lot number of manufacture; the year; and sometimes the propellant.

SWN stands for Swynnerton, the factory where the cartridge was filled. I presume that 4P is the lot number and 49 is obviously the year. 1Z indicates Mk 1 version with nitro rather than cordite filling. H.E. is obvious, which (if I’m right so far - I’m no expert on these) just leaves the “1.R.” to be explained.

Hi Tony,

Thanks. I had noted propellant and date aspects and had suspected that SWN was Swynnerton. I guess I’m just a little surprised that the actual load would be indicated (HEI) as the projectile markings would do this. I wonder then if this side stamp was added after the round was loaded. It would explain the difference between the ‘48’ in the headstamp and ‘49’ in the side stamp.

I’ve had a good look under a loop and to me it is definately an “I” as opposed to a “1”.

Thanks and regards,


“H.E.I.R. 1Z” is “High Explosive Incendiary Representative Mark 1Z” and was used to load most of the 20mm alloy case trials. This method of using an inert filling meant the shells had the ballistic performance of the real thing without any of the range safety problems of HEI. The stamp was added when the round was loaded.

The rest is as TonyW says, Lot 4P loaded in 1949 by Swinnerton. The case is probably one of the order for 250,000 cases shared between RG and Kynoch.

I have an article about British alloy case trials in the next IAA Journal.


Thank you TonyE. I look forward to your article!