I've actually fired one, back about 1970. About as accurate as a well thrown baseball. Firing at night looks like a tracer round. The back blast vents out the holes in the side of the barrel. Once clear of the barrel, you get a puff of hot gas in the face, but not bad.

I always wanted somebody to use the gyrojet projectile on top of a std. brass case with a fairly light propellant load fired from a rifled barrel. The ballistics would be a mite hairy...

Not responsible for advice not taken...