I recently came across some 9mm ammo headstamped WW 6 44. I was not aware our military was using 9mm this early. I wonder if it was to test some German weapons captured during the war?

Lend-Lease to our Commonwealth Allies for their Stens and Hi-Powers?

United Defense M42 - Wikipedia

This page from W. H. B. Smith's "The Book of Pistols & Revolvers" shows some velocity and penetration tests of various 9mm Parabellum (Luger) cartridges out of a Walther P-38 after WW2. Note the first and second ones : Remington Commercial and Western War Contract. Perhaps yours is the second one? The two American loads and the two standard German pistol loads (slightly hotter) from 1918 and 1941, all seem to be running at today's 9mm NATO velocities.

Attached Images

HPWhite9MMtest.jpg (62.4 KB, 22 views)

From “The History of Modern U.S. Military Small Arms Ammunition, Volume II, 1940-1945 (page 228 and 9):

“On April 2, 1942, the Ordnance Department adopted the 9mm ball pistol and submachine gun cartridge…. The round was designated Cartridge, Ball, 9mm, M1. It was shown on DWG. B257589, dated April 20, 1942. This round was procured by the Ordnance Department primarily for the Office of Strategic Services who used this caliber ammunition in a conversion for the M3 submachine gun. The M1 9mm Ball would also function in most foreign pistols and submachine guns chambered for this caliber. Most of the contract manufacture of this round was with the Winchester Repeating Arms Co. and Western Cartridge Co………..”