In 1969, has a newly minted tank commander out of the NCOCC at the Armor School, we were familiarized with the use of the 90 mm main gun on the M48A3 in the indirect fire role, using a gunner’s quadrant and the gunner’s azimuth indicator. We had aiming stakes and took directions from an improvised Fire Direction Center, with firing tables for the 90 mm M17A1 HE round, the fuse of which could be set for point or delayed detonation.
I believe the maximum ordinate would yield a maximum effective range of 16,000 meters or so.
We were told that this was an inefficient use of tank guns, due to the relatively small bursting radius of the 90mm HE round and the accelerated wear on the gun tube, but could be employed in an emergency.
We actually used this technique one time on the DMZ, when responding to a request from a beseiged fire support base. Each tank in our company fired all 15 rounds of our HE load. Most of our main gun load was canister, beehive, WP, and HEAT.