The Broad Arrow
No doubt you have observed the so-called "broad arrow" stamped or otherwise marked on military equipment and pondered its meaning.
The symbol, or 'mark', was used to denote Crown property as early as the reign of King Richard II (1367-1400), but to Gunners it has added significance. On ordnance not only does it denote Government ownership but it indicates the piece has been successfully test-fired, ie it has passed proof, or has been proved.
Having been used by the Board of Ordnance since earliest times, the term 'broad arrow' is probably a corruption of the "Board's arrow".