will m16 mag fit sr 556
Yes. Ruger SR 556 lowers accept standard STANAG (“M16” or “AR-15”) magazines.
can lake city? 308 win brass be reloaded
Yes. Lake City brass is good stuff. However, military brass tends to be a bit thicker and so has smaller internal volume, so it’d behoove you to reduce your loads by about 10% when switching from commercial to military brass, then testing with a chronograph while making adjustments. Be particularly aware of signs of high-pressure (see your loading manual) when using military cases. Reloading military cases is perfectly safe, so long as you’re careful and realize that they’re slightly different from commercial cases.