Note: in AM, use 'double-barreled'
1.
A double-barrelled gun has two barrels.
...a double-barrelled shotgun.
ADJ : ADJ n
2.
A double-barrelled surname has two parts which are joined by a hyphen, for example ‘Miss J. Heydon-Smith’. ( BRIT )
ADJ : ADJ n
3.
Double-barrelled is used to describe something such as a plan which has two main parts. ( JOURNALISM )
ADJ : ADJ n