The Biggest gun in the West

Just after the Second World War the west was facing a problem, it's the same problem that tank designers face today. Its gotten to the point where a tanks protection is better than the guns on tanks, they are lacking the power to perforate the tanks defensive layers. Just after the war we were worried about the IS-3 , and later about the T-54.

Jim Warford uploaded these pictures over at Tanknet, they show an IS-3 that's been shot up by L7 APDS... Scared yet?

Normally what would happen, and what happened during the Second World War, is that a gun would defeat an armour, then the other side would increase the armour, so the gun's increased by a small amount so that they could defeat the armour. Then the cycle would repeat. So you got a steadily increasing calibre of gun in small increments.
In the early 1950's the chief engineer (I suspect his name was Lillywhite, but I haven't been able to prove it yet) at the Fighting Vehicles Development Establishment sat down, and thought 'What if we cut out the small steps and went directly to the end, to the biggest gun possible? What is that calibre?'. So he pulled out his slide rule and some paper and set to work. The figure he came out with for the maximum theoretical gun calibre was 180mm. This was the 180mm Lillywhite gun. The engineer also calculated the estimated performance of the gun. It fired a whopping 71.5 lbs AP shell at 3720 feet per second. This gave a kinetic energy at muzzle of around 20 megajoules. In comparison a modern 120mm L/55 smoothbore with the best available ammunition is providing about 13 megajoules (APFSDS). The Lillywhite had such a big round it was split into two bag charges and the projectile.
From this point the gun was developed and became the 183mm L4, that we all know and love on the infamous FV215 and FV4005. The L4 had a single bag charge, but the projectile was very similar. It however lost some of its velocity as it was only ever designed to fire HESH rounds (HESH rounds are often seen as "low velocity"). Well these low velocity HESH rounds were still able to generate about 18 megajoules of kinetic energy.

"I say, you! Over there in the tank that looks like an inverted frying pan... Yes you! Want some low velocity HESH rounds delivered?"

The L4 was worked into the FV215 and there has been a great deal of misinformation floating about this tank in modern games. It could carry twenty rounds, of which twelve were ready rounds. The turret could rotate through 360 degrees but the gun was to be locked out and prevented from firing if the barrel passed forty five degrees of arc. However the gun could be fired when pointing backwards.
From the outset the army was lukewarm about the FV215. When the Malkara guided missile appeared on the scene they got behind the project with enthusiasm and dropped the L4 as soon as they could. Then the L11 120mm gun showed up and it had enough power to defeat the enemies armour and things settled down.

In the 70's people began to see armour once again getting better, and forecasts indicated that Russian tanks could get very scary. During the time when I was growing up a lot of writers and commentators pointed out how superior the Soviet armour was, going on about how invulnerable their tanks were (much like people do today in regards to the T-14), so once again the idea for the next generation of tank guns showed up. Of course after a few years we actually learned their armour was pretty poor. First on the scene was the 152mm for the MBT-70 project. Not much is known about this gun, but from little that is known is that it'd have produced a kinetic energy value similar to the 120mm L/44 smoothbore with its earliest variants of ammunition.

There's even less known about the gun that came next, it was 145mm joint US-German gun project hinted as the "Future Armament system" on one sketch. It occurred sometime about 1986. We do however know what it would have looked like as some models have survived.

The next gun to be developed was the 140mm FMBT gun, and is widely fitted to a whole host of tanks. It was a NATO standard weapon in many respects. The choice of 140mm wasn't as random as you might think, research shows that above 140mm the projectile is actually less efficient with that calibre being the optimum. Coming in two parts it had to be screwed together before use.  As you can see the sheer size of the rounds would have meant a autoloader was necessary, as it'd be like trying to load a small human into the breach with each round. The 140mm FMBT gun developed a whooping 20 megajoules of energy.

I got bored, and did some drawing, ably helped by Maddest. Who now wants me to add All of the shells ever made since 1945 to the diagram.

Finally we come onto modern times when the Germans announced that the 120mm was no longer good enough, and that presumably not enough power could be pulled from the gun to defeat current threats. Rheinmetall has designed the 130mm Main Gun Combat System. The MGCS is reported by Rheinmetall to have 50% more energy than their 120mm gun. However one has to be very careful about these sorts of claims as most companies sales teams are worse than the shadiest of used car salesman. An expert who has to deal with this sort of stuff professionally laughed at that figure and suggested its more likely to be around 40% at best. This would give the MCGS around about 15-17 megajoules of energy.

Classic German design, at its finest, draw a box around the gun and call it a Panzer! Then lie about its emissions.

Without a technological leap guns are going to have to become bigger if you want them to keep punching through enemy armour. However that will impose a series of big problems that modern western armies are reluctant to have on their tanks, such as limited ammunition and less crew. All things considered the immediate future of tank armament is looking very tumultuous.