Will the buffer and spring in this work properly with a 9mm dedicated upper and lower? If not which buffer would work best. — 3 Answers

This model has a 3.3oz buffer weight ideal for a 556 Nato carbine. For a 9mm, you will need a heavier buffer weight, around 5-8oz. You need to choose the model that has a 7.6oz buffer weight. You would think a pistol caliber build, which uses smaller rounds overall, would require a lighter buffer weight. The opposite is true, though. In a pistol caliber build, the traditional gas system of an AR is missing outright. Furthermore, the bolt carrier group is typically heavier. To smooth the action and ensure reliability, a larger buffer weight is a must-have.

Standard buffer will work but you'll need the pistol spring (not the standard).