Re: Does it matter?
In general, machine guns (M-60, etc) have more generous chambers to aid feeding and extraction when dirty, so fired brass will have expanded a bit more than if it had been fired from a tighter chamber, like most bolt guns have.
So most people avoid machine gun fired brass, as there is a greater chance of case head separation after several reloads.
Brass fired from something with tighter tolerances (but still more generous than most bolt gun chambers) like a SR-25 or other quality "gas gun" should not really be an issue.