All rounds for the gun were 5.9kg, with MV of 823 m/s [2700 fps] so penetration would be superior to US Army 3-inch ammo; Rich has some details on this, maybe with penetration figures, which might be found in Campbell, Naval Weapons of WWII.
Yes, somewhere I think I have the comparative penetration values derived from the postwar Navy tests at Dahlgren in 1947. However, the epiphany was not the penetration, it was the Navy finding the "3” M62 APC split and broke up when impacting 3” homogeneous armor plate at 30°and 60°obliquity at velocities up to 2,900 feet per second. On examination, the projectiles exhibited variable hardness with soft spots at the nose, contributing to the failures. However, Navy 3” Mark 29-2 AP projectiles did not shatter." On top of that, while the Army Ordnance did react to there own, earlier tests in 1943 to improve the hardness specifications for 90mm AP that resulted in the (too late for the war) development of the AP T33, no effort I know of was made wartime to improve the specifications for 3" M62 APC or 90mm M82 APC. Nor did it seek to solve the problem with premature firing of its BDF found in the Showburyness tests of May 1944. Instead, Army Ordnance went with HVAP as the solution and ignored its problems.
They did have development programs for a number of weapons to improve general APC design over the war.
APC T29 redesign of M61 around spring/summer 1944
Objective of increasing the wall strength to the rear of the driving band, 75mm APC M61 is being redesigned with a narrower explosive cavity and smaller fuze seat. Gave better penetration then standard M61. Design of shell near identical to 3" T17E1 so testing halted while work was done on that.
as the relatively low velocity 75mm is going out of fashion, project was terminated in 1945.
APC T17 redesign of M62
Project to improved performance of the M62. Reduced diameter and increased length of explosive cavity. mechanical failure in shell body resulting from peculiarities in heat treatment from one manufacturer, an investigation was instituted but project ended up being closed 1945
APC M82 trial of M68 B.D. fuzes
A trial happened at Aberdeen on 3rd June 1944 to verify accuracy of British report on fuze failure in 90mm APC rounds.
a number of rounds were tested at various velocities to simulate ranges against a 4 inch plate at 30 degs. One partial failure out of 9, the rest functioned fine.
APC M82 increased performance
Original intention to achieve MV of 2850 f.s. using M2 propellant, had to change when excessive erosion using M2 propellant observed. A switch was made to M1 powder giving a velocity of 2800 f.s. with high density loading required for propellant ignition difficulties with M23 primer resulting in switching to T33 primer, T33 was thought to be mechanically weak and not standardized resulting in T36 primer.
Penetration of homogeneous armour at 30 deg at a range of 2100 yds. is claimed to be increased from 3.7" to 4.25" as a result of increase in velocity from 2650 to 2850 f.s. summer 1944
APC T25, T25E1, T26, T27, T28, T28, T35 and T39
Designed with the aim to improved the performance of the M82 against 4" and 5" plate at 30 and 20 deg. Only the T26 was better then the M82 against the 4" plate possibly due to it's greater weight. Against the 5" plate all designs were superior except the T27 compared to the M82. The T35 was a 24.1 lb AP projectile fitted with a ballistic cap similar to that used on M82.
T25 was redesigned (T25E1) with single radius ogive but having a bump/knob on the nose of the cap in order to provide same thickness of cap formerly used with the blunt nose T25.
T35 and T25E1 ammo batch produced with W.D. 4370 steel and given modified heat treatment to give extreme hardness at the point tapering towards the base, firing tests showed inferior performance to M77
Conclusion reached that a solid shot would give overall better results leading to the design of the T39, a solid round with tracer and light weight cap and high hard-ability steel. Design had very hard nose and bourrelet tapering off to hardness of 45 Rockwell C at the base.
Batches produced with both hard and soft caps. T39 round designed essentially for the defeat of heavy homo plate at steep angles without sacrificing performance against lighter face hardened plate.