Military Brass is good stuff as long as it was not shot from a machine gun with very loose head space. It is possible machine guns such as the M249 (5.56mm) and M240 (7.62) can seriously malform the brass making it very difficult to size.
Machine gun brass also has the issue with case splitting, where there is a chance your loads will totally split the case in half. But the warning signs of case splitting is easy to spot, you just have to inspect the inside of every case with a flash light. If you see a ring develep on the walls of the inside, throw the case away.
Lucky for us, most of the NATO 5.56 we have access to is not from an MG but but from a M16 or M4.
7.62/.308 is another story though, as most of this stuff is from machine guns. If you are lucky, you got some 308 MG brass that came from a MG with tight headspace, therefore resizing will be easy. I myself got a bad batch as I had a bitch of a time resizing the 308 PMJ and LC brass that I recently picked up.
The primer of 308 military brass is also crimped on super tight. My Hornady and Lee dies could not handle crimp+size in one step, even with excess lube, so I had to buy the Lee decapping die ($10) to decapp first then size. Then I would proceed with just sizing, and even with excess lube for each case, it still took a lot of force to resize.
Lastly there is swaging, I have an RCBS swager and it takes forever! I always tell myself I need a Dillon Swager but as soon as I am done I forget that I need it.
Eventually I was able to clean and size everything, and so far so good as everything reliably went bang.