Dave - I have had a dig in my files (which I should have done in the first place) and as always found some info I did not know I had regarding the case taper and the different bullet diameters.
Source is an Armament Research and Development Establishment report dated September 1956.
First the case taper. The report states:
At Ottowa in 1954, U.K. delegation had considered that the taper on the long cone of the cartridge case of .012 inch per inch was too slow and might cause extraction difficulties. However, the U.S. was not prepared to increase the taper without strong evidence of the advantages of so doing. It was also considered that the tapers of .005 inch per inch and .013 inch per inch respectively on the short and long cones of the chamber were too slow for satisfactory functioning in automatic weapons. They were, however, considered acceptable for the proof weapon chamber for use in ballistic firings; but it not been the intention of the U.K. to use them in service weapons, although, Messrs. F.N. did use them in the prototypes of the F.N. rifle. P,3., therefore in their work to make the F.N. rifle more reliable in the desert, and to produce a satisfactory conversion of the .303 inch Bren L.M.G. to 7.62mm calibre, have selected tapers of .013 inch per inch and .015 inch per inch in the hope of preventing undue interference between case and chamber. Some special cases with a taper of .016 inch per inch are also being manufactured for use with an even greater chamber taper. Trials are being carried out by P.3. in which the weapon/ammunition performance is assessed by the length of bursts attained while firing in a simulated and “standardised” sand storm. The recent modifications by P.3., together with their proposal to introduce a chromium plated chamber, seem likely to give satisfactory functioning of the weapons with the current L2A2 ammunition.
The taper was therefore maintained at the original figure.
The report makes it quite clear that the experimantal taper is .016, and that is what is on the headstamp. I had misread mine as .018.
With respect to the various diameter bullets, the report has this to say:
“It was found that the streamlined lead cored bullet gave poor accuracy in a hot F.N. barrel to N.A.T.O. design (i.e. bore .300"±.0005”,groove .308"+.001") after less than 10,000 rounds fired in the standard cycle."
It went on to say that whilst the N.A.T.O. requirement was that accuracy was maintained at the end of 5,000 rounds fired at 125 rounds per minute,the War Office requirement was for the S.F.M.G. to fire 8,250 rounds in one hour using not more than three barrels
Accuracy trials were fired at 200 yards in new (1000 rounds) and worn (7,500 rounds) F.N. rifles with lead cored bullets of various diameters. The results were interesting.
The service bullet (.3085") gave a group 12 x 15" in the new barrel and 50 x 40" in the worn.
.3105" bullet gave 9 x 12" and 11 x 13" respectively and the .3125" bullet gave 9 x 12" and 5 x 8".
It was obvious that a great improvement could be achieved with the larger bullets, and ballistic firing trials showed that by adjusting the charge pressure and velocity could be maintained at the correct levels.
Many more combinations of bullet diameter, leed size and rifling form were tried and in the end it was decided that the same reults could be achieved by keeping to the N.A.T.O. bullet diameter of L .3075", H .3085" but reducing the bore diameter of British barrels to .296". This was supported by a U.S. report from Aberdeen that a .296" bore would be more satisfactory than .30" but this work had not been followed up.
I hope that explains things from the U.K. end!