We all knew that it was unlikely that an enemy fighter would get us by approaching to within 1500 yards and dead aft of us. If we were to be shot down, most likely we'd get hit during a head-on attack or at the end of a lead-pursuit intercept carefully orchestrated by a ground controller. Oh sure, there was a slight chance that a second pass by an interceptor failing on either of the two primary attack modes might wind up in a stern chase, but that wasn't what we feared.
But because supposedly wiser heads than we ordinary crewmen dictated policy, we dutifully practiced firing our guns over the designated ranges. A couple of times a year crews were required to fly to the gunnery range and fire off 4000 rounds of ammunition. There were no aerial targets on the range, so what were we to fire at? Certainly they didn't want us to shoot at other B-58's, or even at our KC-135 tankers.
The tail gun was radar equipped and scanned a conical region astern the Hustler. The Defensive Systems Operator (DSO), had the gun system controls front-and-center in his aft cockpit. A left side joystick could be used to manually designate targets to be tracked by the radar. There was also an automatic mode where the radar and computer could lock onto the most threatening target aft of the bomber. Then the DSO would arm the gatling gun and get ready to shoot at the optimum range, a mere 1200 yards away.
On the gunnery range, however, there were no closing targets to aim at. No other plane would be foolish enough to approach with range of an armed tail gun. So what in the world was the DSO to practice shooting at. Actually there were two things that could serve as lock-on targets, neither of which was at all realistic.
First there was the convergence of the contrails dragged behind the plane, under many flight conditions. The radar was sensitive enough to "see" the cloud-like contrail. It was possible, on occasion, to get the radar to lock onto that vapor cloud and enable the system to be fired into the ether. But there was another target too, an actual hardware one to be blown to bits by the deadly Gatling gun. It was also possible to see the spent casings of 20mm shells dumped behind the plane.
A short burst fired straight aft, just to test the gun itself, would cause casings and links to go. The radar could see and lock onto the casings and links ejected from the lower-aft ejection door. A quick DSO, and you really had to be quick, could actually shoot at those rapidly receding targets. Not very realistic, was it? More realistic was the training we got in the flight simulator, where electronically generated targets could be put onto the screen for us to acquire and shoot.
This former DSO would have gladly traded the whole tail gun system for an equivalent weight of extra fuel. That might have meant another 30 minutes worth of flying time (subsonic). Ah well, the experts thought they knew better. Hmmmm