Nice little pic of the M1 tank’s Muzzle Reference System.
One of the keys to the incredible accuracy of the M1 is the MRS. Basically, it is a precise measurement of any thermal distortions of the barrel. Firing rounds, or differential heating due to environmental factors can cause minute changes in the barrel, typically a slight droop after firing several rounds. By referencing that distortion, corrections can be made to sight alignment, increasing the accuracy of subsequent shots.
Or you can just watch the fall of the shots, and manually adjust. That’s what us M2 guys did.