Just roughly measured it, am i measuring the whole of the mount, if so its 45mm so therefore i'll need the 56mm, it seems to also be a 1" tube, i'll give the gun shop a call tomorrow, if they have one i'll ask them to put aside til sat so then can get the gun set up on sat and zeroed hopefully, got all day to chuck a bit of lead in the garden, prolly end up on here on sat having probs zeroing it.