Barrel making was another serious "bottleneck" in rifle manufacture and efforts were made to simplify the process. In May, 1941, trials were carried out with barrels having two grooves instead of the normal five. A number were fitted to No. I Mark III. Rifles and were found to be satisfactory and comparable to the normal barrel. Further trials were carried out to establish their accuracy life and whether they developed any tendency to nickelling under Service conditions. Two No. I Rifles with two-grooved barrels were tried against two No. I Rifles with normal barrels. They were fired for accuracy, submitted to a functioning test in which the rate of fire was ten rounds in one minute, and re-shot for accuracy whilst they were still hot. This was continued in cycles of fifty rounds until 500 rounds had been fired through each rifle.
There was no appreciable difference between the two forms of rifling in either accuracy or barrel wear, and nothing to suggest that the two-groove barrel would be prone to excessive nickelling or fouling. It was decided to adopt the two-groove barrel as an alternative form of rifling for all .303-in. rifles. Many thousands of these Mark II barrels were made for the NO.4 Rifles and gave excellent service. To further ease production difficulties it was decided to omit the final operations of fine boring and lapping from the barrels of NO.4 Rifles. Tests had shown that there was no excessive nickelling in barrels from which these operations had been omitted.
