I saw the testing of billets described but cannot now find the source. It made sense to me that it would be done for the reasons I mentioned.
Still, statements that the U.S. did not proof test military rifles are simply incorrect. Brophy, Hatcher, and Ferris all mention proof testing of the completed rifle, and Brophy describes the proof of barrels, receivers and bolts of the M1903 before assembly, stating that if those parts did not bear the necessary proof marks they would not be assembled into a complete rifle. Also, the barrels were proved separately, as barrels (Brophy), and that included barrels used as replacements, since the barrel shop had no way of knowing which barrels would be picked for new rifle assembly and which would be set aside for factory or field replacement.