Under the fateful date of December 6, 194 1, the Armoured Force Board at Fort Knox, Kentucky issued a report of a magazine trial of Thompson magazines, excerpted as follows:

Magazine, 30 & 40 Round for Thompson Submachine Gun ..... Purpose
To test the 30 and 40 round magazines as possible substitutes for the 20 round slip and 50 round drum magazine.

Discussion

A total of 2,000 rounds were fired using the 30 and 40 round magazines. Both types functioned very well from a mechanical standpoint.

The 40 round magazines consisted of two (2) 20 round clip magazines welded together front to front with a 7/8 inch offset on the ends so that the end of each clip can be inserted in the gun. The 30 round magazine is similar to the regular 20 round clip except that it is 2 3/8 inches longer.

The 40 round experimental magazine does not appear to offer any great advantages ..... in that after 20 rounds are fired, it is necessary to remove and reinsert the magazine to fire the 20 rounds. The time required for this operation was from five (5) to eight (8) seconds. A definite disadvantage of the 40 round clip is that when firing from a prone position, the open end of one portion of the clip has a tendency to gather dust and sand in it while firing the opposite side of the clip. The 30 round magazine offers the advantage of 50% more firepower before changing the clip over either the regular 20 round or the experimental 40 round magazine. Either type, however, is superior to the 50 round drum.

Recommendations

The Armoured Force Board recommends that the 30 round magazine be substituted for the present 20 round clips and 50 round drums for the Thompson Submachine Gun for use in the Armoured Force.

A close up look at the existing examples of the prototype thirty-round magazine shows that two Crosby twenty round magazines were used. The top of one magazine was cut off and welded to the bottom of the second magazine. This adds 2 3/8 inches to the length of the body. A longer follower spring was added to feed the ammunition. The magazine also featured enlarged and numbered shell counting drain holes in the side. However, these were not put into production, due to the fear of dirt infiltration in the magazine.