I have found that when running Perfect Paradox with a backup mag, it reduces your maximum reserves by 1 shot. To elaborate, I have a Perfect Paradox with field prep. Without any mods I hold a total of 28 shots. 7 in the mag and 21 extra, total of 28. If I add the backup mag mod, the shotgun holds 8 in the mag and I get 19 extra, total of 27. Is this normal? Usually when I add backup mag, it might increase the mag size but I have never seen it reduce the total ammo count.