Mil-Spec and SAAMI spec are two different things. Somethings but not always the cartridges may be fired in the other gun (.223 for 5.56 mm or .308 Winchester for 7.62 x 51 mm Nato). For a civilian gun, what type of chamber is it? Is it minimum SAAMI spec which would be a match chamber? A go gauge may work in it but not a no-go gauge.
Buying surplus ammo may be affordable, but doesn't mean that it will work in your gun. Remember the military doesn't dispose of ammunition just because they're being nice. They're getting rid of stuff they don't want or need. It may be old. It may be poor performing. It may be substandard and hence dumped onto the market. Caveat emptor.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!