I believe I fired quite a few of those rounds out of the M42 Duster.
https://en.wikipedia.org/wiki/M42_Duster
This has been a long time ago, and I actually have two fired cases from it and I think we are talking about the same thing.
In the early 1980s, I was a member of a National Guard Unit that still used the M42 Duster long after the active duty Army quit using them.
This was MOS 16F and at the time of my involvement, AIT was at Ft. Bliss Tx.
https://www.amlot.org/u-s-army-1421/
I believe this same gun platform was also used as a naval anti-aircraft gun. As you can read at the above link, the Duster was used in Vietnam against ground targets.
The rounds we fired were HEITSD (I think that is what they were called). High Explosive, Incendiary, Tracer, Self-Destruct. We shot them at radio controlled drones that trailed a streamer behind them. As well as at ground targets. Somewhere I took part in a live fire exercise where several Dusters lit up a passenger car. Obviously it was not only completely destroyed but it actually rolled several times.
The rounds were contained in (I believe) a four round clip. There were two guns and two guys loading them. You dropped the rounds into the top where the clip lined up with guides. The spent cases and the clips ejected out the bottom of the vehicle. In practical use, since the guns fired full auto, you really needed two more guys handing the ammo to the two loaders. The ammo came in a steel can that had something like four clips in it. In addition to those guys, you had the gunner and the "Lead Setter". The gunner obviously fired the guns and aimed the guns. The "Lead Setter" sat on the other side of the guns and adjusted for lead: what this consisted of was a mechanical device that physically moved the sights as the "Lead Setter" watched the stream of tracers and he would steer the tracer stream into the target. The gun could be run manually with hand cranks or it could be electrically powered. You also had two sets of sights: one was just a steel ring type thing and the other was a glass plate very similar to a modern red dot sight. You sighted this all in by putting a metal cross hair device into the end of the barrel, then at the rear you put a mirror. You would then look down the breech of the gun and put the cross hairs on a target like 1000 yards away or something like that. Then you adjusted the sights to be aligned at the same place (bore sighting). There was more to it than that, but that is basically how it worked. You also had to do something with a gunners quadrant.............................
Again, this was over 30 years ago and I am somewhat hazy on the whole thing.
