75mm Firefly?

Having just watched the “Inside the Chieftain’s Hatch” episode about the Sherman Firefly - https://www.youtube.com/watch?v=__Y8YJeas4I – I’m amazed they did so well. (And full of admiration for their crews) But this did make me wonder about an alternative.

The original plan for the A27 Cromwell was to install a new Vickers 75mm HV gun; however, at a late stage it was found this would not fit. While I haven’t found a great deal of information about this, it appears to be similar to the later 77mm but using US 75mm shells and designed for an external mantlet.

Can anyone point me towards further information, either on-line or print? I’m wondering how far design had progressed and would it be feasible for this gun to replace the 17pdr in Fireflys. This might allow for more space internally, retaining the bow gunner and a better all round tank?

I don’t expect this to be a game-changer - for example, it could lead to logistical issues, mixing QF 75mm and 75mm HV, .30 Browning and 7.92 BESA - but an easier time for Firefly crews (and possible acceleration of the Comet) would certainly be worthwhile.

(Edited due to slight senior moment about Shermans and BESAs)
 
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Nice idea, like your thinking. In AN ALTERNATIVE 1930s BRITISH TANK GUN - AND ITS SUCCESSOR by Anthony G Williams at http://www.quarryhs.co.uk/alt WW2 tank gun.htm he mentions it briefly. Read the 3rd paragraph from the end for his comments; and a picture of the ammo. By the way anything written by this gentleman is very interesting especially his book http://quarryhs.co.uk/TFWreviews.htm The Foresight War which is an amazing Alternate History book; although I am sure most people on this site will have read it.
 
A G Williams' site is a mine of useful information.

Unfortunately, the designers of the 75mm HV and the Cromwell dropped the ball. And the Army didn't want external mantlets. (Except when they pinched them off wrecked Shermans to fit out NA75 Churchills) But if there was time to develop the 75mm HV for volume production* and they could fit in a Sherman turret there would have been no need for 17pdr Fireflys. (And possibly a higher number of British tanks able to take on the Cats - and still with a reasonable HE shell for infantry support?)

*I wonder how long it took to redesign the 17pdr to fit Firefly? Turning it through 90 degrees, altering 6pdr recoil system to suit, etc?
 
Making my way through this thread - https://www.tapatalk.com/groups/war...-he-shell-size-for-wwii-tank-t37949-s500.html - maybe an alternative is for the British to develop a 75mm APDS round for the M3/QF 75mm. A G Williams posits a 75mm shell using the same core as the 6pdr APDS with similar penetration while retaining the HE capability of the M48 shell.

USA Ordnance developed a HVAP for the 75mm, but never deployed
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I was thinking of a mixed company with M3 75mm and 75mm HV armed tanks. But they had the "same" issue with Shermans and Fireflys so as you say, not a problem. I didn't know about the 75mm HVAP. I know there were tests of 75mm APDS, but this seems not to have been an American thing, whereas the British went down this route with the 6pdr and could possibly have done the same with the QF 75mm?
 
I'm no ballistics expert, so this is a SWAG but...

The OQF 75mm has a m/v of 620m/s with a 14.62lb* M61 shell

The QF 77mm has a m/v of 785m/s with APCBC and a penetration of 137mm at 500M. This is getting pretty close to penetrating a Panthers frontal armour of 80mm at 55 degree slope - roughly 140mm equivalent and will certainly make a mess of a Pzkpfw IV or Tiger.

The QF 6 pdr with L/43 barrel goes from 853m/s with AP to 1151m/s with APDS - an additional 300m/s.

I wouldn't expect the same gain from the 75mm, but with APCR/HVAP or APDS* could you reasonably anticipate an extra 150m/s or so, pushing it into 77mm territory?

*Note - the 17pdr APDS "core" weighs 7.7lb.
 
Well...what started as an attempt to get a smaller* Vickers 75mm HV gun into a Sherman/proto-Firefly has morphed into...something else.

It seems the University of New Mexico did some experiments in 75mm APDS starting in 1943. These seem to have been discontinued because HVAP was on the horizon and also due to issues with plastic sabots expanding with moisture; however, using a 57mm shell as a core, they got up to around 850m/s. This should certainly match the performance of 6 pdr AP - 112mm penetration at 500m. Would it have matched the 77mm for a similar m/v with APCBC? Seems unlikely, but I'm no expert.

But as APDS was used by the British in 6 pdr and 17 pdr flavours, I have to wonder why they didn't produce a 75mm version with a dural sabot, keeping the benefits of using HE shells but giving Sherman, Churchill and Cromwell crews more of a chance in tank v tank combat?

*Well...smaller than a 17 pdr.
 
Well...what started as an attempt to get a smaller* Vickers 75mm HV gun into a Sherman/proto-Firefly has morphed into...something else.

It seems the University of New Mexico did some experiments in 75mm APDS starting in 1943. These seem to have been discontinued because HVAP was on the horizon and also due to issues with plastic sabots expanding with moisture; however, using a 57mm shell as a core, they got up to around 850m/s. This should certainly match the performance of 6 pdr AP - 112mm penetration at 500m. Would it have matched the 77mm for a similar m/v with APCBC? Seems unlikely, but I'm no expert.

But as APDS was used by the British in 6 pdr and 17 pdr flavours, I have to wonder why they didn't produce a 75mm version with a dural sabot, keeping the benefits of using HE shells but giving Sherman, Churchill and Cromwell crews more of a chance in tank v tank combat?

*Well...smaller than a 17 pdr.
Just a guess, but, I'd put my money on the same reason they didn't continue the development of the 75mm HVAP round, at least in the US. "Tanks don't fight tanks." Idiots!
 
I can sort of see why the US didn't push for 75mm HVAP (Devers), but the British had more of a history of tank v tank combat and retained a certain number of 6 pdr Churchills fairly late in the war. They certainly had 6 pdr APDS in Normandy and it doesn't seem that much of a stretch to have 75mm versions for the tanks armed with either US or British 75mm guns. I'm not sure when they started development, but as per Wiki The Edgar Brandt engineers, having been evacuated to the United Kingdom, joined ongoing APDS development efforts there, culminating in significant improvements to the concept and its realisation. The APDS projectile type was further developed in the United Kingdom between 1941–1944 by Permutter and Coppock, two designers with the Armaments Research Department. I'd do a timeline if I was anything of a writer. :(
 
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