MBO, the mob usually just buys their guns on the streets so they take what they can get, at possibly the lowest price. The DeMeo crew was a little different because Roy was a bit of a gun nut and an expert at many military weapons, so he knew what he was doing.
The rest of them just took what they could get, usually a .38 does the job very well. I could see them favoring semi's because of a larger capacity magazine and faster rate of fire, but all in all, you're right about the revolvers.
Don't forget though that semi-auto's can be fitted with suppressor or pre-fitted like the .22 Ruger, which is one of the mob's most prized weapons for silent hits.