What can a 9mm do that a .38spl +P can't do just as well or better?
The LCR can shoot 9mm +P also. So it delivers about 50% more energy and penetration than 38 +P.
Not really true. While .38+P pressures are considerably lower, case volume is higher and when using appropriate powders there is very little difference in velocity for a given weight bullet.
For example, SAAMI +P pressures are 38.5Kpsi for the 9mm Luger but only 20Kpsi for the .38Spl +P. SAAMI references 1235fps for a 115g in the 9mm Luger +P and 1150fps for a 130g in the .38 Spl +P. Hodgdon lists .38 Spl +P loads of 110g/1337fps/17.7Kpsi and 125g/1228fps/18.3Kpsi.
The higher pressure of the 9mm +P loads will increase wear and tear on the revolver but you really don't get much in return in terms of downrange performance - if anything.
Not saying you shouldn't use the 9mm or that it doesn't make sense in some applications - particualrly if you have other 9mm Lugers and want to have common ammo. But for most people most of the time the .38 version is a better choice.
