.50 Cal is small potatoes in 40K, it seems unlikely that they can kill a Space Marine in anything but the unlikeliest of circumstances.
I mean, I can see that, theoretically, it's possible to pull off a golden BB to the optic nerve somehow... against guys who can see bullets moving in midair and react to them by slapping them... maybe the Space Marine just found an iPhone and he's so appalled by Twitter he loses his will to live. But in theory, if every single factor went right, maybe a .50 cal with explosive bullets can get that perfect hit to an SM who took his armor off.
How many times is that going to happen though? Space Marines will not decide to retreat and leave Earth alone because one guy got killed. There's no reasonable way this convoluted set of circumstances are going to happen more than once or twice. "People are shooting at you" is just not an Out-of-Context Problem for Space Marines.