Springfield Armory tested the .220 Swift during Fiscal Year 1936. The report was titled “Test of ‘Swift’ Rifle, Cal. .220, and Ammunitions for Same, Manufactured by Winchester Repeating Arms Co.” DTIC and NTIS do not show this in their collections. Perhaps a search of the National Archives will yield better results.
While stationed in Corregidor (Philippine Islands) during 1935 and 1936, Frank T. Chamberlin (US Army Medical Corps) conducted an unofficial series of lethality tests, pitting the .220 Swift against Army mules in a variety of scenarios. Chamberlin wrote up an account of the testing for a magazine at some point. (“American Rifleman” perhaps? I am not certain as to which issue. Anyone know?)
Starting in 1950, Donald L. Hall of Aberdeen Proving Ground’s Ballistic Research Laboratory experimented with the .220 Swift as part of a study on infantry rifle effectiveness. A 60 grain .224" bullet roughly homologous in shape to that of the issue .30 M2 ball was used in the experiments. The test firings were performed by William C. Davis Jr. and Gerald A. Gustafson of Aberdeen’s Small Arms and Aircraft Weapons Section. The final report, An Effectiveness Study of the Infantry Rifle,is part of the DTIC/NTIS collection.
Also part of the DTIC/NTIS collection are several reports in which the .220 Swift is used for bore erosion experiments with different barrel liners and coatings. In certain cases, M60 and MG3 GPMG are even reworked to fire the .220 Swift.