Anti Armour 20mm casing Need help with its identity

Hey guys, I normally just casually collect old stuff don’t really have a focused collection. I most get by doing my own research but this one has me totally stumped. To me it looks like a fired 20mm casing. I never heard of using spent casing to make new projectiles.

The paper wrapped lead? rod is seat in to the neck and is .799-.803. The steel “driving bands” on rear of the casing are about 1.805. The copper washed? steel tip is about .780ish. The weight on my scale is 1 pound 5ozs. My thought was the whole thing looked to well done to be trench art but seemed to light to be a real projectile. If any one has any ideas please chime in.,rxJjm9T,EsSEo9K,fLzx2la,6dLF4Vg,GqYI90G#0