How was the brass shipped to you? If it was in a air tight ammo box or sealed plastic bag it may be OK. If not I would only size about 10 cases, reload it and shoot it over a chronograph. If the shots hold a very good average velocity OK. If the answer is NO, For safety run them through the rifle fire the primers and then treat them as once fired brass.
I take it these shells were for sniper training Vietnam era or 1000 yd. competition and do not have crimped in primers so it should be pretty straight forward sizing and de-priming these cases. Good Luck
I've bought some of that brass and loaded it. Just dumped in powder and seated a bullet. Neck tension varies but for what I needed it for it was fine-shot about as well as anything else.
My question is why is the government buying all that ammo, they are selling demiled ammo in calibers that the military never used. 458 Lott?, 300Savage?,7WSM?,300WSM?, 7RM?, 300RUM???
I have some new (unfired) Federal 1993 Gold Medal Match brass that was de-milled (bullets pulled and powder dumped). A buddy bought about 250 pieces, and left about 100 of them here, and told me to keep them. So I loaded up a few for ladder-test loads. Never got around to testing them, then I found some Norma .300 WM brass, and ordered it.