I always use to carry my full size autos in a shoulder holster, but I had a situation a few years ago where I thought that since the gun was vertical holster I didn’t need the retention strap and I ended up almost losing a gun when I unexpectedly felt it almost fall out.    I’ve grown to appreciate locking holsters like the Blackhawk Serpa and I always use a retention strap when using a shoulder rig.   Shoulder holsters are frowned upon by many firearm instructors because it’s hard to train someone on a firing line in that configuration.   Back in the day, most of the FBI agents I worked with carried in a shoulder holster, but not anymore.

There have been times that I have walked up to a firing line and noticed that I could clearly identify the hollow point ammunition in a firearm because even though it was holster, I was looking down the barrel.   A few guys in a long range shooting competion had their guns in gun holsters, and they were locked in, but I was still uncomfortable standing there.  If you have a horizontal shoulder holster and you were on the firing line, and I was standing directly behind you, you would be pointing  your gun at me without having your hand on it.   Is this alarming?  Not really,  but there is a split second period where if you were to go and unholster the gun, your hand would be on the firearm as it was pointing at me, and you would be violating one of the safety rules.