All it takes is one quick change in the law & you have lost a section 58 anyway, Seven cartridges that previously were regarded as section 58 obsolete calibres were recently removed from that list, there was a six month transition period after which pistols chambering them they moved from sect 58 to section 5 or 7(1)
They could be retained on an fac if old enough.
I wonder how many pistols of those calibres simply "disapeared" in that transition period?.
I wouldnt worry about it, if you have a section 58 rifle or shotgun you want to shoot & have a slot for it on your ticket go ahead.