As family crests go, the garbe (also garb or wheat-sheaf), banded, is a distinctive one in heraldry terms. That's the good news.
The bad news is that a lot of families are noted as using it, or using variations of it, as per Fairbairn's Book of Crests of the Families of Great Britain and Ireland, 4th Ed., London, 1905.:
The fact that it is within a circle (roundle?) may help narrow it down. Knowing the actual colours of the family crest would be best, but impossible in this case.
I'm afraid that doesn't narrow it down very much at all. Do you have any information on the history of the gun, past owners?
Beautiful gun, by the way!