Except the US didnt produce a lot of HVAP ammunition, whereas Britain had a decent amount of 17 pounder ammunition for their various 17 pounder tanks (late war) and AT guns.
17 pounder and 76mm were somewhat similar... Depends on what they were mounted on and the variation of the gun. 17 pounder shells were a bit heavier, a bit larger, and had a bit higher muzzle velocity... But that didnt mean it was necessarily better given RoF and accuracy for a 76mm... Given tanks with those cannons used APCBC ammunition.
Hastily looked at forums.
I'd leave British weapons to the Commonwealth - when they come around.
17pdr APDS or early sabot rounds had horrendous dispersion/accuracy issues as the subcaliber bullet would usually yaw due to imperfect separation of the sabot. Which is why Sabot rounds don't become standard for every army until finned dart penetrators are used in the 60s. Every one else took the lesser penetration at range but the ability to hit by selecting subcaliber rounds still encased in the the jacket aka HVAP, and Russian/Soviet APCR rounds.
To actually hit something with a 17pdr or a 77 you need to fire the APCBC shot. Tests by the US army at Isingy france in 1944 with UK gunners netted a 57% 17pdr APDS hit rate at ranges from 600 yds to 300yds . . . 17pdr APDS was a white elephant. Again there is a reason that every other combatant relied on APCR/HVAP; the need to actually hit the target.
British Army’s own test results, obtained by two 17pr-armed Sherman Firefly tanks in September of 1944, were no better. These results and comments were published in Warrant Office Reports WO 291/1263 and WO 165/135 dated 22 September:
400 yds
APC hit 90.5%
APDS hit 56.6%
600 yds
APC hit 73.0%
APDS hit 34.2%
800 yds
APC hit 57.3%
APDS hit 21.9%
1000 yds
APC hit 45.3%
APDS hit 14.9%
1500 yds
APC hit 25.4%
APDS hit 7.1%