An interesting story but, like so many family stories, a bit muddled.
As KGarrad observed the Arms were granted to Rev. John Hockin, Vicar of Okehampton, in 1764.
The Arms are quite "busy" for the period and I suspect were designed on the basis of the story of the actions of the Reverend's ancestor at Gwithian, thus the inclusion of the musket.
This would have been a standard grant from the College of Arms and therefore subject to the usual rules of usage. No special conditions, I'm afraid !
If you are a direct descendant of the Rev'd John in the male line, but not the senior line, you could use the arms with some form of differencing : perhaps change the main background colour from red (gules) to another colour, or change the fleur-de-lys in the base to some other pattern. If you intend to do this it would be best to at least consult with the College of Arms to ensure that you do not duplicate someone else's Arms by accident.
As an aside : you need to be very wary of the back stories provided for many of the Arms in the older reference books particularly with the lesser gentry. Burke's Landed Gentry is notoriously unreliable and even some of the Heralds Visitations contain flights of fancy.
If you really want to be sure of your ground with the Hockin arms the best thing would to be to apply to the College of Arms for a copy of the Grant. (But it will cost !)
Maec