.303 inch Observation
Observation rounds were intended for use in training Vickers gunners in long range fire where the fall of normal ball rounds could not easily be seen. The Observation rounds were intended to give a good puff of smoke when striking that could be easily seen by the gunner and instructor. They were not intended for operational use in the field.
Observation Mark I
"Cartridge S.A, Observation O Mark I" was initially issued in the 1920s but was not formally introduced until List of Changes Paragraph A.9091 dated October 1934.
The case was a normal Berdan primed case with a black primer annulus. The headstamp included the code "O I".
The bullet was similar to the Ball Mark VII except that the aluminium tip was replaced by about 5 grains of a mix of phosphorus and aluminium powder. The tip of the bullet was pierced with a small hole which was sealed with solder. This was intended to assist the break-up of the bullet on striking the target. The bullet had a black tip for identification and weighed 174 grains.
The propellant charge was 36.5 grains of Cordite MDT 5-2 to give a velocity of 2,400 fps.
Observation O Mark I drawing from Kirkee Ammunition Pamphlet No.1 dated January 1945.
Indicating Mark I
The Indian Army did not consider the O Mark I to be suitable for use in tropical climates, presumably because of the danger of leaking phosphorus in storage so had designed their own Observation round. This differed from the British design by having the phosphorus sealed inside a steel container in the centre of the bullet, and was tested in the United Kingdom in 1931. A later modification of this design using white phosphorus contained in a gelatine capsule inside a steel container was adopted by India prior to WW2 as the Indicating Mark I.
“Cartridge S.A. Indicating .303 inch Mark I” was approved to design KF 575 in the late 1930s. The title was later changed to “Cartridge S.A. Observation .303 inch O Mark II”. It had been declared obsolete by 1945.
The case was a normal Berdan primed case with a black primer annulus. The headstamp initially included the code "IND.I"' but later examples used the code "O II".
The bullet weighed 174 grains and had a lead tip and lead rear core. Between the two was a tinned steel plate case in which was inserted a gelatine capsule filled with a 3.25 grain phosphorus pellet. The bullet had a black tip for identification.
The propellant charge was 36.5 grains of Cordite MDT 5-2 to give a velocity of 2,400 fps.
Observation O Mark II drawing from Kirkee Ammunition Pamphlet No.1 dated January 1945.
Observation Mark IIE
In 1941 India adopted another observation round, the "O Mark IIE”. The meaning of the “E” suffix is not shown, but as the bullet was constructed similarly to the British O Mark I with the phosphorus in the tip it has been postulated that the “E” stands for “English”.
"Cartridge S.A. Observing .303 inch O Mark IIE" was approved to design IA.7 in 1941. The title was later changed to "Cartridge S.A. Observing .303 inch O Mark III"
The case is a normal Berdan primed case with a black primer annulus. The headstamp initially included the code "OIIE" but later examples are marked "O III"
The bullet has a rear lead core with a 2.5 grain phosphorus pellet in the nose, replacing the aluminium tip of the Ball mark VII bullet. It is not an exact copy of the Brirish O Mark I as instead of having the hole in the tip filled with fusible metal, the envelope is internally thinned on the ogive to aid break-up. The bullet weighs 174 grains and has a black tip for identification.
The propellant charge is 36.5 grains of Cordite MDT 5-2 to give a velocity of 2,400 fps.
Acceptance proof was that at 500 yards 95% of bullets should give a good puff of smoke when striking the target plate.
Observation O Mark III drawing from Kirkee Ammunition Pamphlet No.1 dated January 1945.