What is Comet’s gun?

It’s a 17-pounder, right?

Definitely looks like a 17-pounder, doesn’t it?

Well – looks can be deceiving – it is a 77mm.

Next Question: what is a 77mm?

Black and white photograph showing Comets of the 5th Battalion RTR, 11th Armoured Division - thought to be somewhere in North West Europe, 1945 — A Comet of 5th Battalion Royal Tank Regiment, 11th Armoured Division.

Britain’s Premier Hole Puncher

While the 17pdr is without a doubt a fantastic hole-puncher (to use the informal British term for a gun able to defeat German armour) able to deal with all but the heaviest panzers.

However, it did have one rather important downside – it was really very big. 458cm long and 2.1 tons when on a towed carriage. A full 2 metres longer and half a ton heavier than a 6pdr. This was necessary of course – it had to be long to give the round the velocity required to penetrate the 118mm of armour at 1000yds (231mm if using APDS) it was capable of dealing with.

Black and white image of British crewmen loading a shell into the breach of a 17 pounder anti-tank gun — The breach of an early 17pdr anti-tank gun

The result of this (for tank design anyway) was that there were very few tanks actually capable of mounting it; just look at the modifications required to get the thing into something even somewhat close to a Cromwell hull.

When it came to Britain’s new cruiser tank, the gun was quite naturally a rather important aspect of it’s design. Quite naturally again the gun at the top of the list was the 17pdr. One issue; even with the increased diameter of the Comet’s turret ring, yet again it wouldn’t fit.

A New Gun on the Block

At the same time, Vickers were instructed to develop a high velocity but, most importantly, more compact hole-puncher. This was to be a brand-new High Velocity 75mm weapon. It was a very good bit of kit.

But then it got better. Someone had the stunning idea of slightly increasing the calibre to 76.2mm (3in – the same calibre as the 17pdr). This allowed for the use of rounds from the 17pdr while exploiting any advances in the more powerful weapon’s ammunition natures and simplifying production.

Black and white image of a Comet tank on the factory production line - in the final stages of having its gun barrel being lifted into place. — The 77mm barrel being fitted.

The 76.2mm projectile was mated to the casing from the original HV 75mm – which was also the same casing used in the 3.7in heavy anti-aircraft gun. The result of this allowed for a much shorter overall length of round but only a small drop in cordite capacity.

Pair of black and white photographs showing tank crews loading ammunition into a Sherman Firefly and a Comet tank - showing the difference in size between the rounds. — These two images demonstrate the difference in size between the larger 17pdr and shorter 77mm rounds.

What’s a Millimetre Between Friends?

So if this new gun has a calibre of 76.2mm, then why is Comet armed with a 77mm gun?

Surprise! its the same gun!

It is purely a naming thing. It was to stop any (rather inevitable) confusion in logistics circles between the 17pdr, the American 76mm and 3in guns plus the 77mm – which have calibres of 76.2mm, 76mm 76.2mm and 76.2mm respectively.

Black and white image of a Comet tank at Lommel Ranges in June 1945 conducting firing excersizes — A Comet of 1RTR at Lommel Ranges, June 1945

Was the effort worth it?

The next question had got to be if it was any good. Surely a gun which on the surface appears to be a compromised 17pdr must massively suffer in its performance?

In short: no, it was actually rather good.

Using APCBC, it could penetrate 110mm of armour at 1000yds. Compare that to 17pdr APCBC which, at the same distance, would get through 118mm. The other massive plus towards the 77mm was something unheard of for a British AT gun – a good HE round was developed for it!. To be fair it took a while and went through a couple of less successful incarnations but the end result was a round packing 600 grams of HE. Compare that to the 480 grams of the 17pdr and 667 grams of the Sherman’s 75mm – the latter a gun designed around having a good explosive round.

Black and white illustrative diagram showing an APCBC round, as used by the Comet tank. — 77mm Armour Piercing Capped Ballistic Capped (APCBC)

Black and white illustrative diagram showing an APDS round, as used by the Comet tank. — 77mm Armour Piercing Discarding Sabot (APDS)

Black and white illustrative diagram showing a High Explosive round, as used by the Comet tank. — 77mm High Explosive Super High Capacity (HE/Super/HC)

All in all, the 77mm was a really good gun. A good balance of penetrative power and size, while also being a truly dual-purpose bit of kit. The 77mm was a critical part of making the Comet the best British tank of the Second World War and the western allies first tank that was truly a match for the German big cats.