|Watervliet Arsenal Museum - Tank Cannon / 02 XM-150E6 152mm Gun Launcher Breech
02 XM-150E6 152mm Gun Launcher Breech
This is a cutaway of the breech for the either the XM-150E6 or the M81E1 152mm gun/missile launcher of an M551 Sheridan light tank. It is designed to fire both conventional spin stabilized projectiles and infared guided missiles. I wondered how a missile could be fired out of a rifled barrel without spinning upon exiting the barrel. Well, the answer is in the photo below:
A key way slot is cut in the barrel that runs the length of the barrel. There is a corresponding key on the missile. As the missile is slid in the key enters the funnel shaped part of the slot at upper right and as it is pushed forward the key is centered in the slot and ready for firing. As the missile is fired the key in the slot keeps the missile from spinning as it travels the length of the barrel. Initially the key way was cut deeper than the grooves for the rifling (which I think this barrel has too). Unfortunately after firing less than 100 rounds of conventional projectiles the rifling and key way were damaged enough to make the barrel beyond acceptable tolerances. The solution was to make the key way the same depth as the rifling which doubled the life of the barrel to 200 rounds, which was still not a very good performance. I believe this is a prototype with the deeper cut key way.
Ellis S. "Scott" Hamric, Director of the Third Cavalry Museum in Fort Hood, Texas wrote me with this very interesting account of the reliability of the 152mm Shillelagh missile:
"I was looking at the great photos on your Watervliet site and noticed that the 152mm Sheridan gun tube page lists the Shillelagh missile as being wire guided. These missiles were infrared-guided rather than wire guided like the TOW missile. That was one of the problems with that system. We never knew when something in the tracking and guidance system would fail and cause the missile either to turn around, fly straight up, or do some other crazy thing. I saw one at Fort Bliss, TX when the 3d Cavalry was stationed there in the 1970s make 180 degree turn and come back past the range tower, pass through the peak of a GP medium tent and ricochet off of the engine cover on a Gamma Goat truck before falling to the ground and breaking in half. Needless to say, the folks in the tent and the mechanic working on the truck were scared out of their wits by the event. "