Taking FAC air rifle off ticket

[Shootist]

Apologies of this is in the wrong section. It is a question specifically about the law. Here goes.

A person has an air rifle that is registered as a S.1 firearm on his FAC. All nice and legal. The question is, does the law permit it to be 'detuned' (if it was tuned in the first place) and then removed from the FAC or is it "once a S.1 always a S.1."

This part is important, "please show your workings." At risk of sounding like some of the snottys, I know that some police forces allow this to be done. I know that some people have managed to do this. I know that some people know somebody who was talking about it the other day and they said their firearms enquiry officer said it was OK. What I need is the law on the matter. I've had this shown to me before and it was quite clear, but a tad indirect that it is "once a S.1 always a S.1" Trouble is, I've lost it.

Asking a Force licensing officer of their underlings is rarely satisfactory. They will usually just reply what amounts to "Because I say so." There is an appalling lack of knowledge among such people. So, I'm hoping a fine tuned legal beagle can tell me the law on this.

[Aggy]

When I asked I was told 

if the gun started out as a factory made fac then it can’t go to sub 12

but if it was a sub 12 to start with and was converted to fac then it can go back .