Why? Muzzle velocity wouldn't pierce anything.
More importantly, as a howitzer I doubt it even had AP rounds as an option.
since the gun on the lvt is a 75mm pack-howitzer, designed for close-in artillery support and not direct anti-tank use, so no AP available. great for taking out towns though.
Towed and airborne versions of the 75mm howitzer in Europe had HEAT rounds for use against armor.
But since the LVTs were primarily used in the Pacific where enemy armor was not generally much of a threat, I don't know if issuing HEAT rounds to the LVTs would be appropriate.
LVT4s were supplied with AP rounds in the Pacific. The problem is that some are equating using AP rounds against other armored vehicles but in the Pacific, the LVT4s were primarily used to support ground troops along with the Sherman tanks. Like the Shermans, the LVT4s were used to take out reinforced enemy positions, like reinforced bunkers where the AP round was able to penetrate through the feet of concrete and steel. However, there were some occasions where the LVT4s did fire on Japanese armored vehicles using AP rounds but the primary use was to take out hardened targets.
The mounting of the turret on the LVT4 also allowed it to be used in the direct fire role.
AP rounds were available. As you said, they sucked, but they were available.
Whoever said they sucked doesn't know what they were talking about. LVT4s firing AP rounds were very effective in taking out Japanese strong points as exampled in battle such as Peleliu, Saipan, Iwo Jima, and Okinawa and a myriad of other island battles for the Pacific.
I can find no data to support your claim. Only things to shoot out of this 75mm M3 was the M89 Smoke & M48 H.E. rounds. 
One source would be "Brotherhood of Heroes" by Bill Sloan.
ack-ack
