Talk:Vigilante Armaments/@comment-100.2.124.254-20171030012122/@comment-2606:A000:8115:200:A07A:D263:A0F3:3976-20171118012740

All sentinel weapons have a 'type', from which they equip certain mods. I suspect that none of them are actually classified as primaries, but only as their basic type (such as rifle or shotgun), which would be why you can't equip Primary mods on them.