gun carriage

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A frame or structure upon which a gun is mounted for firing or maneuvering.

from The Century Dictionary.

• noun The carriage or structure on which a gun is mounted or moved, and on which it is fired.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A metal frame on which a piece of ordnance is mounted for firing or transportation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a framework on which a gun is mounted for firing