I believe it's some iteration of this series of 110mm ammo development, third from left in this picture, which LooSeR posted before:
Xlucine explained this to me over teamspeak. Remember how an L7 is basically just a 20 pounder with a wider bore and the cartridge case necked out to 105mm? The Brits tried to get lightning to strike again by necking out the case one more time, 'til it was completely straight-walled to make a new 110mm gun. The idea was to have something that would be an easy drop-in upgrade for the L7, just like how the L7 was an easy drop-in upgrade for the 20 pounder. Since basically all of NATO except the British were using L7s at this point, the potential sales were enormous.
Guns are basically piston engines that throw their pistons down range, so going up in caliber is basically the same thing as boring out the cylinders in a motor. But the difference in swept volume between a 105mm tube and a 110mm tube isn't enormous; a little less than 10%. So to get more performance they started running up the pressures. This led to the cases sticking and not extracting properly, so they went to a semi-combustible case design, which is what you see above.
As the gun was developed further, the case stub got smaller and smaller until it went away entirely and the design was switched to bagged ammo. Only thing was, by this point the pressure was so high that the breech obturator ring design from the L11 would not work and they had to come up with something better. This breech design was recycled in the 120mm L30 on the challenger 2. So there you go; despite using the same HESH ammunition, the L30 is actually a development of the L7, not the L11.
Like most ideas in British tank design, the 110mm EXP-7/EXP-14/EXP-whatever was obsolete by the time it was ready for prime time. The British were completely, stubbornly convinced that rifled guns firing APDS were the way to go, well into the 1970s when everyone else in NATO had stopped dicking around with gun-launchers and figured out that smoothbore and APFSDS were the correct choice. As usual they were way behind the Soviets, whose T-62 design had a smoothbore gun, had already seen combat, and was already in the process of being replaced with a next generation vehicle!
You can also see that it uses two-piece ammo, which is basically the way to go if you're firing APDS. It also looks like some of the propellant charge lives with the sabot. Very Soviet.
Oddly, this exact diagram shows up as a generic diagram for an autoloader on the army-guide article on autoloaders.
I take it back, that's not propellant living with the sabot. That's two rows of big propellant charges. It's totally unclear to me from that picture where the projectiles live. Is that smeared blur near the middle of the carousel the sabot? Is this like a double-stacked T-72 autoloader? If so then how the hell did they plan on shooting their beloved HESH rounds?
British tank design is Kurt Cobain; occasional spasms of cleverness amidst a corpus of generally mediocre work that is for some reason praised to the heavens. HESH is their Courtney Love; a weird dalliance from the past that they have indecorously and inexplicably held to that is holding them back and making them do stupid things. Also both British tank design and Kurt Cobain are dead.
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