I have a clutch of wheelguns that shoot semi-auto rounds; 9mm, 10mm, .40 S&W and of course .45 ACP. The short auto cases frequently are easier to eject fully from a snub with a short ejector rod, particularly true of 9mm and .45. Of course both of those rounds come in a whole lot of loadings, different bullet types and so forth, so selecting the right round for your purposes is easy. The 9mm is a nice snub load and you can get mild or fire breathing loadings from lots of ammo builders. The 40/10mm revolvers can really put out a hot round, particularly in the Smith 610. The 646 is an easy carrying service size gun due to its lightweight cylinder.
The real advantage for a service or HD revolver in an auto caliber is the quick reload with moon clips, also you can get some flexibility in caliber in the same cylinder, I have a 627 with 3 cylinders, with one cylinder I can shoot .357 Magnum, 9mm, .38 Super, 9X21mm, 9X23mm, .356TSW and even .380 auto-if one wanted to shoot such a lightweight out of a 627. I even have a Browning revolver (a Barracuda) that has interchangeable cylinders in .357/9mm.
My usual duty weapon is a Browning HP so my 940 makes a perfect BUG, carried in the off side pocket it draws easily from a pocket holster and can be reloaded if necessary by using the HP mag to feed the rounds into the cylinder-of course without a moon clip you will have to pick the empties out individually. Usually I carry 2 moon clips in a single small speedloader holder stacked one over the other.
The S&W 547s do not need nor can they use moon clips, they have an interesting extraction system that works well and HKS makes speedloaders for the 9mm for that gun. The 3 inch 547 makes just as great a carry gun as its K frame relatives the M10, 64 and 65.
I am currrently having an M60-4 and a 37-2 fitted with 940 extra cylinders, the 37-2 cylinder will be refinished to match the gun, thus giving me a lightweight J frame in 9mm, which is not otherwise available. I would do a 637 too, but I was able to get only 2 extra 940 cylinders.
The 37-2 just may replace the 940 as BUG and off duty revolver. Lately 9mm has been cheaper to shoot that .38 special, so I can get in range time even through the ammo crunch, which appears to be loosening up some lately.