Which nail gun for fencing job?

Hi,
I've just come across this group today and was wondering if anyone can offer some advice. We are about to put up a 2m high close board fence, 14m long. Not a particulary big job, but having shoulder problems at the moment I thought I'd buy an air coil nail gun. I have a 120psi compressor. Looking on ebay I saw a good price on a Rockworth air coil nail gun firing 25-50mm nails, diameter 2.1 and 2.5. Since I would be fixing 16mm thick boards to 30mm boards I thought this gun with 45mm nails would do the job. As I've never actually used one before I mailed the supplier and asked if it would be suitable for this job. I received a reply which simply said "Not suitable". Can anyone please tell me why it wouldn't be suitable, and what would be. Thanks.
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