Unless the projectile is rocket propelled, I cant see how a tube that short could possibly accelerate a mass sufficient to send it out 1000 miles.
Need my bs meter.
If you read the article, it assumes a guided, self-propelled projectile.
Of course, the round for extreme ranges will include additional propulsion and terminal guidance. A prototype munition exists but I do not know if it has been fired.
BTW, the first picture in Popular Mechanics is a display piece at Ft. Sill, OK. 1950’s era weapon.
The article provides a possible explanation.
They are speculating the “shell” is ramjet-powered. Ramjets are more fuel-efficient than rockets, since they use air instead of carrying their own oxidizer, but need to be going above Mach 1 to work. They speculate the gun gets the shell up to speed, and the engine takes over.
So the gun is really a launcher for a ramjet missile.