For some reason the Americans never addopted it.
There was a fairly convoluted sequence of events that prevented the Firefly from coming into US service.
Perhaps the heart of it is that it is not easy to work a "foreign" design into service without top-down coordination meeting up with bottom-up need. In this case the coordination was weak, and the need was intermittent at best.
The US had no domestic facilities to built 17pdr guns, and US Army Ordnance had no interest in starting production (see further discussion below). But the British offered to do conversions for the US Army. USAETO was in fact interested in getting some. But …
The British offered to do conversions, but quite fairly enough they offered to do the conversions only after they had finished the conversions for their own needs, and they offered to do the conversions only on Shermans provided BY the US Army (not making use of the stock of British lend-lease Shermans).
The Firefly conversion was designed only for the M4 or the M4A4 Sherman versions, and the US Army did not operate the M4A4, leaving only the base line M4 as a candidate for converting. By the time the British had finished their first wave of Firefly conversions for their own needs, there were no M4 Shermans available in USAETO stock to turn over for converting. US standard was M4A3, but both M4 and M4A1 were "alternate standard". I believe all, or almost all, of the US Army units that went ashore in the June/July/August Normandie campaign were equipped with M4 or M4A1. (The units equipped with M4A3 started arriving in UK in August, and in France in September.) Losses in tanks were greater than the US Army had forecasted, and so there was a very high demand on in-theater stocks of M4s and M4A1s.
As M4A3s started to arrive in numbers, a stockpile of M4s was in fact collected to be turned over to the British for Firefly conversions.
However by the Fall timeframe the British Army upped their own requirement from 1 Firefly per troop to 2 Fireflies per troop, so once again the US conversions fell in line behind British needs. By late winter / spring (early 1945) the offer was again extended to do conversions for the US Army, and in fact some few hundred Firefly conversions were completed, but by this time the USAETO was no longer interested in Fireflies, and they were never issued or deployed.
From what I understand, the 17 pder was tested by the US, but while it offered great AP performance, its accuracy had a bit to be desired. It was tested along with the M1A1 and the new 90mm, and it was determined that both these weapons had much better shot grouping at longer ranges, the 76mm was "good enough" and the 90mm was well into development, and AP performance was close to the 17 pder, with much better shot grouping…
Yep.
A bit more detail …
The 17pdr was a bit less accurate than some US guns, as measured by test fire groupings. But it's higher velocity reduced the impact of range estimation errors, an important consideration in combat that was not well examined in test firings at the time. Still, the 17pdr with standard AP shot was accurate enough by US standards.
But the barrel life was quite short. US Army Ordnance standards were for 1,000 shot barrel life as a minimum. The 17pdr barrel had about a 120 shot life, due to the very high chamber pressure. Ordnance didn't accept that at all. This led to some very interesting memos from USAETO back to Ordnance, urging the development or acceptance of superior anti-armor performance even if it shortened barrel life. Ordnance eventually got the message, and relented on the barrel life issue. But only after the firing tests done in-theater in July / August 1944.
While the 17pdr AP shot was accurate enough, the APDS shot was an entirely different story. In the case of both the 6pdr and 17pdr, test firing reports from this period have long lists of excuses for why they couldn't hit the target on a range at a pre-sited distance. Even the British tests included guidelines for using the APDS only at short ranges because you couldn't expect to hit a tank-sized target reliably at more than about 500 meters. We often lose site of how desperately inaccurate early APDS was. Certainly over the decades the issues were resolved and it became the most accurate projectile available, but when first introduced there were numerous design issues in the ammunition, in the guns, and in the gun sites, that all prevented these rounds from being accurate.
On the other hand, the US 76mm M1A1 was an exceptionally accurate gun, beyond even what other US guns achieved. And the US APCR ammunition was exceptionally accurate ammunition.
Testing demonstrated that the US 76mm with APCR was about as capable against armor as 17pdr with standard AP shot. 17pdr with APDS could not reliably hit the ocean from a boat, and so was rejected. So given the choice between getting new guns with new ammo, or just getting new ammo, the US went with getting new ammo.
This is not to say the US APCR was without flaws … most notably the early US APCR had a higher tendency to ricochet from sloped armor. This was addressed in several upgrades to the APCR round, finally being resolved in the very late war / post war APCR ammunition, which performed very well against T-34-85s in Korea.
Testing demonstrated that the US 90mm with standard AP shell or AP shot was about as capable against armor as the 17pdr with standard AP shot. And the US 90mm with APCR was about as capable against armor as the 17pdr with APDS. With the standard AP rounds the accuracy between the US 90mm and the 17pdr was negligible, but the accuracy advantage of US APCR over British APDS was overwhelming. And the US 90mm gun had substantially better HE performance, and longer barrel life, than the 17pdr. So for the next generation of tanks after the Sherman, the 90mm was viewed as the better choice.
It was all reasonable enough.
By September US units began receiving APCR ammunition for 76mm and 3-inch guns (in embarrassingly small numbers at the start), and US TD units began receiving 90mm armed M36s.
Small comfort to the Sherman crew facing a Panther in July of 1944, but there was no path to resolving his troubles in July, short of taking tanks out of British formations in favor of US formations, and that was not going to happen.
-Mark
(aka: Mk 1)
