In 1979 timeframe, the Armament Lab at Eglin AFB in Florida received a research task from the Army to examine the freeflight characteristics of long rod penetrators. There was a range at Eglin that was particularly suited for this work. The test gun was a Soviet 76mm Anti Tank gun rebarrelled to take the round pictured below:
This is a 1945 vintage 75mm case with a solid steel neck piece to reduce the caliber to 30MM. The projectile was a long 30mm steel rod that was held in place by four set screws in the mouth of the steel neck insert. My understanding is that there were either 3 of these cartridges made, or three that survived the test program. I saw the gun but have no idea what happened to it. This is probably the longest 30mm cartridge ever tested!!! That is a 9mm Para cartridge in the photo for scale.