Joseph, it would be wise to get a 9mm magazine for reliability with 9mm ammunition as well Sir.

you would need a 9mm mag and the proper recoil spring for a 9mm

Joseph - The barrel is all you will need to convert the pistol to 9mm. The 40 mags can be used for 9mm, however depending on the spring strength of each individual mag, the 9mm rounds will likely start popping back out before the mag is full, due to their smaller diameter. I would plan on getting my 9mm Glock mags if I was going to use the 9mm barrel a lot at the range (to save $$) .I think you will like this barrel. They are noticaebly more accurate than the Glock barrels - at least the ones in the Gen 1 - 4 Glocks.