When COL Payne is reprimanding PVT Johnson about her failing the drug test, she has her hat on. Members of the US Army do not wear hats indoors. When he's finished he salutes her and she returns his salute--that's backwards. She should salute him first and hold the salute until he returns it.
Sheriff Charlie Wade is armed with a Colt 'New Frontier' revolver, first introduced in 1961 by Colt as an updated version of the old Colt 'Peacemaker'. The events in the film take place in 1957 and 1958.
When Mikey and Cliff are in Mikey's sculpture studio, around 55 minutes in, Cliff holds up a fired .45 bullet that had clearly pancaked against a smooth, flat surface and he asks, "What's a pistol slug doing on a rifle range?" Pistols are fired all the time on outdoor rifle ranges, even if not in official events. Also, at the beginning of that scene around 54 minutes in, the camera pans over a collection of mostly spent cartridge cases, including many pistol cases for .44 Magnum or .45 Long Colt, .45 ACP (including one live round with a modern jacketed hollow point bullet), and more.