Thanks for reaching out Tim! I'm afraid there's no 100% reliable way to estimate a round's muzzle velocity out of a different barrel length. The only way to tell for certain is through actual chrono testing. That said, if I had to venture a guess, I'd say this round's MV out of a 18" barrel should be close to 2,775 fps – but please don't hold me to that. To answer your other question, this kind of hollow point bullet isn't actually designed for terminal expansion. Its nose cavity is only in place because the core was poured through it while molten, so it could harden to form-fit the jacket. That promotes stability, like you say!