I was putting in a variation for a couple of things a good few years ago and got the FAC Semi Auto refused as I hadn't provided a letter from the landowner to support my need for it. I wasn't asked for the letter as evidence nor was I explained why I hadn't got it added on my FAC, I had to ask why not. I was pretty annoyed as it then cost me £26 a few years later to submit another variation when I could be bothered again! When I did put back in I asked for vermin conditions and got them. A lot of FAC semi autos are now issued for 'avian pest species' which would not cover rabbits, so be specific in your application that it must be for those species. Once I had it I asked the FLO whether I could shoot foxes and he said that as I had been granted it for vermin, not for vermin and foxes, technically I could not shoot foxes!
Anyway, got a benelli M2, fantastic gun, although it will not cycle 2 1/2 inch cartridges, but will cycle 67 mm cartridges! Will cycle ANY load as long as 67mm or above, I have tried as low as 24g and no problem!
As for magazine, I was under the impression that the 5 shot could not be increased to a 10 shot (or thereabouts depending on size of cartridges) without having it sent back to GMK, but the 10 shot could be reduced to a 5 shot.