Howdy, Pard,
I presume you are referring to military garb worn during the Indian Wars campaigns. You need to decide if you are portraying an enlisted troop or officer, and then whether to portray field "dress" or garrison. Military field clothing is much easier to do, as things were pretty "loosey-goosey" for both officers and enlisted. Until it just plain wore out, many officers and enlisted who had served in the Civil War wore sack coats and trowsers (sic) left over, saving the regulation stuff for wear on their post. The vagaries of weather and other conditions, as well as what the individual could afford/scrounge up, dictated what you'd see. Wide-brimmed straw hats, purchased from the sutler's store, or felt hats usually replaced the official kepi in the field.
Lt. John Bourke, Gen'l Crooks aide wrote, "We looked more like a band of brigands than a military organization!" referring to the Big Horn & Yellowstone Expedition of 1876.
Doug McChristian's "The U.S. Army in the West-1870 - 1880" and his other books, "Uniforms, Arms and Equipment - The U.S. Army in the West 1880-1892", Vol. 1 & 2, are some of the best sources for information on this subject. Doug includes photos that don't just show the regulation stuff, but plenty of the "real world". It sure doesn't look like the uniforms in a John Ford military western!
Welcome to our "mess" (the term used for a group of soldiers who ate together...not our current condition!
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