Thanks to all the members of the NDA Discussion for their help in this article.

Whenever you read about modern tanks then the term "Chobham armour" comes up time and again. The way the term is generally used makes it sound like all Chobham armour is equal. Its not. Its also very hard to prove or research, as most governments have spent a considerable time keeping the exact composition a secret, some with less success than others. So this little article may contain much that is speculation, or picked up from bits of information that is fragmentary.
First things first, Chobham armour isn't an accurate term, it's like a family name for modern composites. It's often used by the Press to describe the concept if not the exact detail to its readers, nearly all of whom couldn't tell a Tiger from a Sherman reliably. Composite armours are nothing new. In the 1930's Vickers designed some of its tanks with thin layers of high quality armour plate over thicker layers of much softer quality armour. Or in World War One some British tanks were tested with oak planking as backing to their steel armour. If you push back as far as the medieval period, chain mail and the padded jacket was technically a composite armour. However the post war composites were generally designed to defeat warheads, such as siliceous-core armour, which was great against HEAT warheads but was pretty useless against kinetic energy rounds.
T-72 Glacis plates, with their siliceous-core insets. The Polish army did look at removing the inserts and putting a more advanced armour type in, which would probably have been called Chobham by the press, even though it had no direct link.
And now for some speculation, there are a few files in an archive here in the UK which are still closed under the national security clause. They're part of Project Prodigal, and come from the atomic weapons research establishment and are talking about defeating projectiles. The interesting thing is the date on these files starts in 1950. It may well be this is the birth of modern composite armours. Anyway the first official work on what was to become the first of the Chobham family, then named "Burlington" started around 1960 or 1961, although the exact date is hard to pin point. Over the next fifteen years a lot of research was done. In 1971 the British started to share some, but not all, of their research with West Germany and the United States. The data given to Germany had an immediate effect on their new tank design, the Leopard 2.
Leopard 2 prototype before Burlington...

...and after.
As the new German tank, and the new US tank, the XM-1 neared entry into service the British were considering their position. Firstly they were worried that the designs of the new allied tanks, especially the Leopard 2, would give away some of the secrets involved in the armour. Equally there had been several security leaks by the allies. British Intelligence also reported that even the Swedish Army had found out some details of Burlington and were considering it for the tank due to replace the S-Tank.
Finally the British were about to start construction of phase 3 FV4030 Shir tanks for Iran. This was equipped with armour called Pageant, although that seems to have been identical to Burlington. The size of the construction order meant that more and more people would be exposed to the secrets. All these factors combined with the risk of losing the prestige of this development meant that the British decided to make an announcement on 15th June 1976 to NATO about the special armour. The day before they decided to give Iran, Germany and the United States a warning that they were going to make the announcement, to prevent them from stealing the British thunder. The following day there was to be a press release.
Something missing from this M1? Maybe the armour inserts have been removed?
For the briefing they test fired some rounds to demonstrate the effect of the armour. A 152mm HEAT warhead, 120mm HESH round and a 120mm APDS round were all fired at a slab of Burlington. They also fired the same rounds at an identical weight slab of normal steel. Against the normal steel all rounds penetrated, against Burlington only the APDS created a slight bulge.

One of the earlier mentioned security breaches is rumoured to be a sample of the armour stolen from a West German lab in 1975. Its rumoured that elements of that sample influenced the T-80B's armour. However one big difference is the T-80 doesn't have the square sided look of modern western MBT's, so its unlikely it's the same armour.
Other changes include the US taking their version of the special armour and adding in layers of depleted uranium, there have been at least two upgrade packages in this. It also explains why US tanks have been heavier than their British counterparts. The British themselves continued to develop Burlington into Dorchester armour.
Nowt to do with the article, just a gratuitous tank photo...
There's one last thing to say. There is another secret file that is closed to the public sitting in an archive here in the UK. From the title it's a project designed to defeat Burlington armour. The title implies it somehow turns elements of the armour against itself to aid in its destruction. The project lasted from 1972 to 1979.

Of course all the above is likely to be in part wrong. As wrong as the experts who are quoted in this 1980's article on the M1 tank. Its well worth a read, just for some of the Chrysler responses.

Image credits: and