arm-chest 

1. A box placed on the upper deck, or in the tops, to contain a ready supply of rifles, pistols, or cutlasses.
2. A similar box or chest used in the military service for the transportation of small arms.

In the book "A Tale of Two Cities," Charles Dickens writes, "So the guard of the Dover mail thought to himself, that Friday night in November, one thousand seven hundred and seventy-five, lumbering up Shooter's Hill, as he stood on his own particular perch behind the mail, beating his feet, and keeping an eye and a hand on the arm-chest before him, where a loaded blunderbuss lay at the top of six or eight loaded horse-pistols, deposited on a substratum of cutlass."

arm-chest by FFFeatherfox February 23, 2019