Here is something I have on the Streamlined “SL” Match round, I have so much info these days I am not sure of the source but it is a good bit of info on the round
.303 Match SL rounds
these are match rounds and could only be used in NRA Match Rifle competitions, not ordinary Service Rifle (b) target shooting.
The bullet is very similar to the military Ball Mark VIIIz bullet with a boat tail, although strictly speaking it was the other way round, with the Mark VIIIz being based on the Kynoch match bullets of the 1930s.
S.L. of course stands for “stream-lined” or boat tailed.
These rounds were first “officially” introduced for the Match Rifle competitions (i.e., at 900, 1000, 1100, and 1200 yards) in 1933, before that date the .303" Magnum round was used. The bullet in the 1932 Magnum cartridge was tried in the “ordinary” .303 case, and as the ballistics were found to be satisfactory, the “Magnum” case was dropped from production.
The 1933 rounds had the headstamp “K33 S.L.”, and came in 10-rd packets with a cream label. The headstamp layout was similar for 1934 and 1935 (obviously with the correct date), then in 1936 it changed to “K36 303S.L.” and similar layout for 1937, '38, & '39. From about 1935 the packets were 20-rds (2 rows of 10) with a yellow label.
The rounds were not produced during the war years, but production re-started in 1946, and carried on to the early 1960s, when the Match Rifle shooters converted to 7.62 Nato.
The load in 1936 was 36 grains of Cordite; I do not imagine it would have changed all that much over the years.
The bullet design did change slightly; the post-war bullet was not exactly the same as the 1933 version, but I cannot recall what the exaxt differences were.
This Streamline round always had the case blackened for about half-an-inch.
Further to my previous post, I have now done a bit more research on this, and pulled a few rounds to see what is inside.
Although I originally stated that the charge in a round that I pulled was cordite, it transpires that the batch concerned were for use by the military in certain tests. The rounds for the civilian market were all loaded with nitro-cellulose.
A round with the h/s K 37 303.S.L. was loaded with 40.5 grains of flat square (or diamond-shaped) flakes, and the bullet weighed 175.3 grains and had a smooth boat-tail, and a cannelure (my 1933 and 1934 dated ones do not seem to have this). The brass-primed case weighed 179.1 grains.
One of the undated post-1946 rounds (h/s KYNOCH 303 S.L.) had 39.3 grains of powder of a cylindrical shape, the bullet again weighing 175.3 grains and with a cannelure, but this time the boat-tail had a distinct “step” to it. The primed case weighed 181.4 grains. (The NRA Journal for 1948 states that the change from flake to tubular powder occurred in that year.)
Finally, I took a round from a 1959-dated packet. This had a h/s KYNOCH 303.S.L., a charge of 39.1 grains of cylindrical powder, and the bullet weighed 175.6 grains, and the step in the boat-tail, although still present, was somewhat less pronounced. The primed case weighed 185.4 grains.
Overall length of each of these was about 3.07".
Some of those made in 1947 apparently have the head painted gloss black (rather than stannic-stained), and I was told that they were intended for the British shooters in the 1948 Olympics.