9mm is fine with decent hollowpoints and my hipower is the most reliable pistol I own. 10mm is too long to fit in the hipower's magwell just like .45acp. FN has produced hipowers in .40 S&W, but you can't upgrade an older model. FN had to change their frame metallurgy to prevent cracking under the additional load. The slide is also bigger and heavier with an additional locking lug.I have one that was 'liberated' at the end of WWII and sold to my father. Fine, accurate, reliable firearm but I wouldn't give you two cents for the 9mm cartridge. I've wondered if It could be given a new barrel and magazine in 10mm . . .