As an alternative, why not try researching local gun clubs. It may be that you can find a member who has the correct licence and access to the sort of gun you want who would be prepared to get involved.
I do not know this but it's possible that you would be able to use such a gun if (a) no ammunition is present and (b) the licence holder is present at all times. At least your local club would be able to tell you.
Failing that you may be able to get away with a "toy" gun for long shots and using the licence holder's hand/real gun for the close ups.
(And, of course, you'd give the club and the individual member credit on the film which is kudos for them and makes your film look a bigger production)