From what many people seem to like about shoulder holsters is the stability of carrying a larger size revolver or duty gun. Something like a Glock 17 or Sig 226, Beretta 96 ect are all a little large for IWB and OWB limits the defensive movement inside a vehicle.

Galco  shoulder holsters come in a very diverse variety. You you use them for concealed carry purposes or for open carry or duty. One of my favorite ways to carry a Sig 226 is a Galco shoulder holster. I like the idea of being able to sit down in a car and still be able to grab the firearm without fiddling with a seatbelt. The only other car friendly variation of carrying a firearm in a car and being able to reach it quickly is with an ankle holster. Galco has several shoulder holster and ankle holsters on the market.