Other than the fact that steel on barrel contact is bad for your gun, they’re very nearly the same performance wise, stick a wad or sabot around it and you’re good to go though.
Being a sphere instead of a cylindrical shape, they’re slightly less aerodynamic than a typical bullet, but if it’s smoothbore it doesn’t really matter since accuracy is what’s going to limit your range.
That’s not going to change a thing, rifling grooves are way too small to engage with the airflow around a bullet, also they’re generally going to get flattened by contact with the barrel. As for engaging like rifling, the harder contact surface is the one that must have rifling, and that’s the barrel. The softer surface (the bullet) is going to get any features it has crushed by the shape of the barrel.
Gyrojets are interesting, the ones that were produced had offset jets to induce spin (hence gyro + jet), which should work in principle, but there’s one interesting problem. It seems they left the barrel so slowly that it was extremely difficult to hold the gun steady enough to keep them on target. Also the slow initial velocity left them extremely vulnerable to crosswinds. So in practice they weren’t actually that accurate.
It does make me wonder if you could use a system more like a RPG or other handheld rocket system where there’s a kicker charge to get it out of the barrel much faster and get it over that acceleration hump it needs to be really accurate. OTOH, Gyrojet was really set on it being a no recoil device, it simplified the hell out of their design, but then again, if you could make a .45 or .50 pistol with a non-locking breech and fires like a 9mm, well you’d have something.
