Tom,
It was a 2.75 Inch Folding Fin Aerial Rocket, that was the diameter of the rocket motor. The M-16 fired a 5.56mm bullet, that is about the same as a 22 calibre shot. I don't think that a rocket with a diameter half the size of a 22 calibre bullet would be very effective, much less be able to carry a 17 pound warhead.
Huey