Bwana:

Not that the difference is all that large, but you're trying to crossbreed apples with oranges.

Since I don't think this has ever been gone over on this forum, here's the short course on Cordite. There were four different kinds of Cordite that I'm aware of. The original, of course, was Cordite (aka Cordite Mk I, adopted by the Royal Navy in 1889); Cordite MD (for "Modified", aka Cordite MDT; adopted by the Royal Navy in 1901); Flat Strip (Kynoch's "Axite" and Eley Bros.' "Moddite"; an attempt to reduce the effects of ambient temp on the burning characteristics of Cordite for use in sporting rifles); and Cordite SC (for "Solventless Cordite"; developed in response to the battlecruiser disasters at Jutland and adopted by the Royal Navy in 1927).

Note that Mk I was adopted by the Royal Navy four years before it was adopted by the British Army. The Royal Navy initially adopted it as a replacement for cocoa (brown) powder, because of the substantial reduction in particulate yield (smoke), for use in main battery battleship rifles, not small arms. Cordite MD was specifically developed for the Royal Navy to reduce bore erosion (by reducing burn temp) in the 12 inch battleship rifles (the barrels of which were only good for 80 - 90 rounds with Mk I), and SC was developed for the Navy to correct the stability issues discovered at Jutland. Cordite Mk I and Cordite MD for small arms were the same as the naval versions - just much smaller extrusions.

The point is that these are all properly called "Cordite" but were different propellants, with different burn rates and temps, and were never interchangable. To wit:

This notation simply records that, in order to obtain the same velocity and pressure with Cordite MD, the charge had to be reduced three grains from the standard charge of Cordite Mk I. This notation is interesting in that it is actually off one grain. For both Kynoch and Eley, 73 grains Cordite and 480 grain bullet was the standard load for the .500/.465 for CORDITE MD. Different powder, different charge weight.

For the conversion formula with the pre-war British double rifles, you go by the proof marks on the flats, absent different information engraved on top of the barrels or the floorplate. These marks were standardized on Cordite Mk I. "Cordite 75" is Cordite Mk I, not MD, not Flat Strip (SC was never used in small arms), unless otherwise noted on the flats. In any event, the conversion formula is for the equivalent of Cordite Mk I only, not MD, not Axite, not Moddite.

The problem with the conversion formula is that too many people don't understand it. The equivalent in Reloder 15 of the original Cordite Mk I charge (equals the Cordite charge X 119%) is a very, VERY rough guide to a maximum load, and is over max in many rifles. Too many guys start with this load. Throw in the fact that 10% density variances between lots of the same cannister grade rifle powders are common, and many of these rifles get badly overloaded.

The best way to do it today is something like the way Kynoch did it as noted above - use late run original (Cordite) factory ammunition to establish a baseline, then start low with RL 15 and work up, a grain at a time, to that velocity. The standard ballistics of the .465 were a 480 grain bullet at 2150 fps from 28" barrels. Original factory Kynoch 480 grain softs loaded in 1966 (Cordite MD) ran 2090 - 2100 fps from the barrels of CFA's new Holland .500/.465. That's about as far as he's got, as we had to abort a range session with handloads due to a chronograph that went tits up. We'll try again soon.
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