USMC T66 "Honeycomb" rocket launchers in action during the Korean War in November 1952

The M16 was a 4.5-inch spin-stabilized unguided rocket developed by the United States Army during the Second World War. Entering service in April 1945 to replace the earlier fin-stabilized 4.5-inch M8 rocket, it was used late in the war and also during the Korean War before being removed from service.

Developed during the latter stages of the Second World War, the M16 was the first spin-stabilized 4.5-inch rocket to be standardized for production by the United States Army. 31 inches in length, it could hit targets as far as 5,200 yd from its launcher. The M16 was launched from T66 "Honeycomb" 3x8 24-tube launchers, M21 5x5 25-tube launchers, and could also be fired from 60-tube "Hornet's Nest" launchers. The United States Marine Corps developed launching systems for the M16 rocket as well, capable of being fitted to standard 3/4 and 2.5-ton trucks. A version of the M16 rocket for single launchers, the M20, was developed as a derivative; practice rounds designated M17 and M21 were also manufactured.

A unit of "Honeycombs" was deployed to the European Theater of Operations in May 1945, and saw limited action in Czechoslovakia before the end of the war; only being used in a single engagement. Two of five battalions equipped with the M16 were deployed to the Pacific Theater of Operations, being stationed on Okinawa and in the Philippines, however the war ended before these units could see combat. The M16 remained in service with the U.S. Marine Corps following the war, with a single 18-launcher battery equipping each Marine Division; these saw combat service during the Korean War, as did U.S. Army launchers, the M16 fired from the T66 launcher being considered one of the "principal artillery weapons in the Korean War inventory".