One of our posters here, Dave Keith, bored a sizing die to .222 and paid/bribed his grandson to size bullets for him. He brought me some at a gun show a while back. I think the original 40 grain bullets were Hornady. You can use Hornet .223 with ease. I've never tried the .224 bullets, seeing no need to. The correct for diameter bullets are still seen from time to time. If you've got a Jet, keep your eyes open for them.
At gun shows, you occasionally see boxes of original ammo on private sellers tables. Often as not, they aren't all still loaded. But the shooters have had it beaten into their pumpkin heads to keep the empty brass. So a box of Jet ammo often has all 50 rounds with only some empty. It reduces the price from outrageous to acceptable.
I bought my original Jet with 10 full boxes of ammo. I asked a buddy if he'd help me lug it all out to the car. He asked what was wrong that I couldn't lug a 22 and a brick of ammo. Then he saw the Jet ammo and understood. I've still got most of those left, along with more I've scored along the way.
Oh, back to the Dave Keith part. I asked him if it was hard to push the .224 bullets thru the die. He said they almost just fall through. I guess its got to do with the small diameter not needing as much effort as a larger diameter bullet. Might also explain why some here just use the .224 bullets with no pressure signs. Not much copper to move. Another trick is to not load up to maximum. Mild or modest loadings are still fun to shoot and still make too much noise. The real key is they don't work the brass too much.