Read description for Extended mag, it was mostly blank aside from its name.
Attached it to my Cx4 Storm, which had 4 or so mods already, the description then became blank for the gun.
Unattached all mods and then reinstalled them minus the Extended Mag.
The gun still had max 22 shots instead of the usual 15, unsure if it had the other mod’s modifications to the stats.
Saving and reloading may have fixed it, either way unloading all bullets and reloading them removed the extra bullets.