Well I'll be the wotsits wotsit of a wotsit !!! You are both clearly right, and I am clearly wrong ... That'll teach me to believe a thread on the BBS, won't it (lol) ???
You see, it was mooted on here many years back that the gun in the scene in question, in the film in question, was actually a Skan, and as this was not challenged at that time, and as said prop gun 'generically' looks a little Skan-like (well, the pistol grip does lol), I -- and possibly many others -- took this as Gospel.
So despite me loving Skan take-down air rifles, the trilogy of Bourne films, and having watched them many many times (but only just now with my eyes properly wide open), I chose (maybe wished) to believe said gun was a Skan ... Oh well, we all live and learn, don't we?
After all, every day is a school day, isn't it? Sorry if I caused any confusion ... Blimey, so its not a Skan air gun. You'll be telling me next that Father Christmas doesn't exist ???