I retired from the army in 1989. Early on, the salute cannon on post was a 75mm pack howitzer fired every morning and evening at flag raising and lowering. use to get the occasional 75mm brass "blank" case when I knew the Officer of the Day. About time I retired or a little earlier, they switched to a blank adapter that used a 10 gauge black powder blank.
Anyone with military contacts may be able to tap into this source. Just a thought./beagle