 Payne replaced the actuator with a heavier one and the recoil spring with a stiffer one; the changes reduced the rate of fire from 800 to the 600 rpm of the U.S. Navy Model 1928.Later M1 and M1A1 Thompsons averaged also 600 rpm.This rate of fire, combined with a rather heavy trigger pull and a stock with an excessive drop, increases the tendency for the barrel to climb off target in automatic fire.