The initial requirement for M18 tank destroyers was pegged at 1,000 vehicles, and when these were completed in January 1944, a second contract was awarded to Buick to manufacture a further 1,800. As mentioned earlier, there was a continual string of modification orders to correct defects uncovered during the Tank Destroyer Board trials. While most of these had been included by the time that the second production contract was awarded, changes continued until the time that production ended. The first 1,350 vehicles were fitted with the R-975-Cl engine rated at 350hp. Starting in March 1944 with serial number 1351, the subsequent M18 76mm GMC were fitted with the more powerful R-975-C4 which had an output of 400hp. A more modest change was introduced beginning with vehicle number 1858 in June 1944: the original gun traveling lock with the swinging lock arm was replaced by a new design using a sliding handle and ball retainer clamp. The initial production batches of the M18 were fitted with the M1A1 gun which could be distinguished by the smooth barrel. There had been repeated complaints about the excessive blast of the· 76mm gun, and tests of a double baffle design were completed in January 1944. Adoption of the muzzle brake was approved on February 11 1944.
Production of the new muzzle brakes did not take place until June 1944. In the interim, the improved M1A1C 76mm gun entered production which was threaded at the end to accept a muzzle brake. The M1A1C gun can be distinguished by a thread-protecting ring at the front of the gun barrel which was added to prevent damage to the threads. The appropriate maintenance work order was approved in October 1944. Besides adding the muzzle brake, the conversion required the addition of three lead counterweights on the end of the gun recoil guard to compensate for the weight added at the front end of the gun barrel. In the event, few of these vehicles appear to have been retrofitted with the muzzle brake during the war. The 76mm guns manufactured with the muzzle brake were designated as 76mm gun M1A2 and these were fitted to new M18 tank destroyers during the final few months of production in the summer of 1944, totaling less than 700 vehicles.
There were a number of small changes introduced in the gunner's controls during the production run. For example, the later production vehicles had a M20 azimuth indicator added behind the hand-traversing mechanism which led to the repositioning of the turret lock further forward on the turret ring. One of the lingering problems with the M18 was the shock absorber configuration and in October 1944, there was yet another effort to introduce heavy-duty shock absorbers. The final production batch of M18 76mm GMC in the autumn of 1944 had the wider 21-in. T82 track, along with widened dust-guards and a new dustguard strip along the lower lip of the hull.
Complaints about the poor armor protection on the M18 led to a series of tests to examine various options to protect it. The study was completed in August 1944 and concluded that supplementary selective armor up to a ton would not adversely affect mobility. A kit was designed which added an armored roof over the turret, but there was no production to retrofit vehicles in service.
