FAC airgun is a great tool and to me more enjoyable than .22 rimfire because of worrying about the higher chance of a ricochet, that and the fact I love airguns!
My FAC is purely for airguns and the application form where it says what cal you want just write FAC airgun, thats all I did and it was fine on my application. Its not much of a pain but just a bit of waiting that is frustrating. As long as you have been well behaved and have suitable land then I cant see there been much of a problem. On my application I needed another FAC certificate holder to take me out, watch me use there gun