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.303 inch Explosive

Whilst the Buckingham designed phosphorus rounds were true incendiaries, the only other round to bear the title "Incendiary" in WWI is more accurately described as an explosive, since it was designed to detonate on striking the target.

Brock VIIK Mark I

Designed by Wing Commander F.A.Brock, RNVR, a member of the famous fireworks family, the Brock explosive bullet was first developed in .45 inch calibre using a .45 inch Express rifle. It was trialled by the Admiralty and they sponsored the development of a .303 inch version for use by the Royal Naval Air Service.

"Cartridge S.A. Incendiary BIK .303 inch (VIIK) Mark I" was approved to design IDW 3418 in late 1916 for Naval service and was not shown in Lists of Changes. The tile was changed in 1917 to "Cartridge S.A. Ball BIK .303 inch (VIIK) Mark I".

It utilised a normal Berdan primed case, and whilst the official drawing showed the headstamp to include the code "VIIK" all known examples have a normal ball headstamp. A blue primer annulus was approved in June 1918 but by then production had ceased.

The bullet had a cupro-nickel envelope with a hole in the nose 0.09 inches wide. The lead antimony core had a .8 inch deep nose cavity into which the main composition was placed with an air channel running through the centre. On top of this was the priming charge which protruded through the envelope and was covered by orange varnish. the bullet weighed 149 grains and had one cannelure. The bullet is seated deeply to give a short overall cartridge length and is secured by three indents low down the cartridge neck.

The main composition was potassium chlorate and the priming mix was potassium chlorate and mercury sulphocyanide.

The propellant was 37.5 grains of Cordite MDT 5-2 with one wad.

Muzzle velocity was about 2,380 fps.

Pomeroy PSA Mark I

John Pomeroy was a New Zealander who came to Britain to demonstrate his explosive bullet in 1914. He was rebuffed at first but returned in 1915 by which time there was an interest in his bullet for Air service.

"Cartridge S.A. Ball .303 inch Pomeroy Mark I" was approved in August 1916 but not shown in Lists of Changes. The title was later changed to "Cartridge S.A. Ball PSA .303 inch Mark I" and again to "Cartridge S.A. Ball .303 inch PSA (VIIA) Mark I". It is also referred to in some official documents as "Cartridge S.A. Ball .303 inch Pomeroy Mark VIIY".

It utilised a regular case with ball headstamp.

The round nosed bullet had a cupro-nickel envelope with a lead antimony core into which was inserted a copper warhead. This was open at the rear end and the nose had a small hole in the front. The warhead protruded from the envelope ans was filled with the explosive composition sealed at the rear end with a millboard disc. The bullet weighed about 155 grains and had a single cannelure.

The composition was a dough of 73% nitroglycerine and 27% kieselguhr with a total weight of 15 grains.

The propellant was 37 grains of Cordite MDT 5-2.

Pomeroy PSA Mark II

An improved version of the PSA Mark I dispensed with the hole in the nose and added a small metal ball to the base of the warhead to improve sensitivity.

"Cartridge S.A. Ball PSA .303 inch Mark II" was approved to designs IDW 3864 and IDW 4525 in February 1917. The title was changed to "Cartridge S.A. Ball .303 inch PSA (VIIAA) Mark II"  The nitro-cellulose version,  "Cartridge S.A. Ball .303 inch NCT, PSA (VIIAA) Mark II" was approved in early 1918.

The case was headstamped with the code "VIIAA" and in 1918 an orange primer annulus was approved, although no such specimens have been seen.

The bullet was similar to the Mark I except it had no hole in the nose and the warhead protruded by different amounts depending on the manufacturer. Two bullet ogives were approved, radius 3.87 for Land service and radius 2.3 for Naval service. The Naval bullet was made standard in June 1918. A small lead shot was placed at the base of the warhead to improve sensitivity. weight was 167 grains.

The composition was a dough of 73% nitroglycerine and 27% kieselguhr, but this was reduced to 60% nitro-glycerine in 1918.

The propellant was 37.5 grains of Cordite MDT 5-2 with one wad or 41 grains of nitro-cellulose.

RTS Mark I

Whilst the Pomeroy was a simple explosive bullet, the bullet designed by Sir Richard Threlfall used a small explosive charge to spread incendiary composition into the target.

"Cartridge S.A. Ball RTS .303 inch (VIIR) Mark I" was approved in late 1917 but not shown in Lists of Changes. the title was later
changed to 
"Cartridge S.A. .303 inch Indicating RTS (VIIR) Mark I" probably to avoid accusations by the Germans of using banned explosive bullets.

The case was a regular ball case with no special code.

The bullet had a cupro-nickel envelope with an open nose into which was fitted a copper tube known as the "trumpet". This too was open at the front and turned over the envelope. A flat nosed copper warhead was fitted into the trumpet. The rear of the bullet was sealed with a lead plug and solder. The bullet weight was about 158 grains.

The warhead contained nitroglycerine in sawdust and the main composition in the lower part of the bullet was white phosphorus.


An improved RTS bullet was designed with a slightly different internal construction.

"Cartridge S.A. Ball RTS .303 inch NCZ (VIIR) Mark IIz" was approved to design IDW 4167 and IDW 4517 in February 1918 but not shown in Lists of Changes. The title was later changed to "Cartridge S.A. Ball .303 inch NC Indicating (VIIR) Mark IIz" and again in 1927 to "Cartridge S.A. Indicating .303 inch  R Mark IIz".

In April 1918 it was authorised that RTS Mark II rounds should have the cases marked with the code "R" and have an orange primer annulus. No such cases have been seen, most 1918 production utilising Kings Norton made cases marked "VIIG" for SPG Tracer.
The bullet had a cupro-nickel envelope open at the nose, but there was no copper trumpet fitted. The flat copper warhead fitted directly into the lead sleeve inside the envelope. The front part of the sleeve held the warhead and the rear part the main incendiary composition.. Early bullets were open at the base and were sealed with solder whilst later production had the envelope covering the whole base. The bullet had one cannelure and weighed between 165 qnd 172 grains.

The warhead contained 3 grains of nitroglycerine in sawdust and the main composition was 8 grains of white phosphorus and tungsten powder.

The propellant was about 37 grains of nitro-cellulose and muzzle velocity was about 2,200 fps.


In 1918 Threlfall together with Col. Todhunter from Woolwich developed a more powerful version of the bullet, known as the RTT. It was not formally adopted but was tested in combat against Gotha bombers and after the war was developed into the R Mark III.

The RTT bullet had a cupro-nickel envelope, open at the nose with a copper warhead containing nitroglycerine inserted like the RTS Mark I. below this was another copper container containing 4 grains of a mixture of Kieselguhr dynamite and lead azide. The main composition was in the rear of the envelope and was 7 grains of "Pi20" above 25 grains of white phosphorus. Pi20 later became known as Penthrite.

The bullet was light, weighing 123 grains and had a muzzle velocity of 2,300 fps at a pressure of 16 tsi.

In late 1918 consideration was given to redesigning the RTT to give a better ballistic shape, resulting in the so-called "milk bottle" design. This was constructed broadly like the RTT but had a narrower copper warhead.

Left: experimental RTT 

Explosive R Mark III

Post WWI small batches of .303 inch RTT were made at Woolwich from 1922 and the design was developed to become the R Mark III.

"Cartridge S.A. Explosive .303 inch R Mark III" was approved about 1930 but never issued for service. A slightly different bullet design was known as "Cartridge S.A. Explosive .303 inch R Mark III* ".

Early rounds had modified ball headstamps but from 1930 the headstamp included the code "RIII" or "RIII*" and some had an orange primer annulus. Some inert filled rounds for firing trials had a "D" included in the headstamp.

There were slight variations in bullet design, but the main one had a cupro-nickel envelope closed over the base and open at the nose. There was no core in the envelope, but a bed of tungsten powder at the base gave weight and above this were two separate increments of composition. The lower consisted of barium nitrate and aluminium whilst the upper was Penthrite. The trumpet in the nose had a detonator of lead azide and dynamite with a flat nosed warhead containing nitroglycerine in woodmeal. The bullet weighed about 128 grains.

The propellant was initially nitro-cellulose but later Cordite predominated.

Left: R Mark III                                                                                                                                Right: R Mark III*