The Norc 5/20 mags ARE slightly different dimensionally than the Gi mags. IF the Norc mags fit your rifle properly [ and this can vary from 14 to 14, but a bit of time dressing the mag release latch surface can be a big help here ] then they can be as reliable feeding as US GI mags.
BUT,
the NORC mags are NOT as hard as US GI mags, and with years [ DECADES?? ] of use, may wear faster.