Re: ONCE fired brass question
Once-fired brass is and always has been a good way to save money to build an initial stash.
Unfortunately you don't know if the first firing was done in a machinegun (7.62) or Squad Automatic Weapon (5.56), where headspace and chambering may have been "Liberal" (sloppy), giving you fat or long cases to work from.
Your best bet would do get some, try small base sizing, and see if it's worth your time and effort. You should get at least three or four firings.
Midway (in Missouri) is probably getting it from the Defense Reutilization and Management Office (DRMO, the military's surplus brokers) at Fort Leonard Wood. Most likely that brass was fired from basic training M16s, with some of it coming from SAWs supporting engineer and MP advanced training.