Options other than paddle holsters
I think the real question that some people need to think about when they are going to take a firearm to a Carbine course is to take some serious thought into what kind of retention they want. I have never seen a firearm come out of a Fobus holster on the firing line, but I have seen gun magazines pop out of that type of magazine holder. Blackhawk makes a better magazine holder due to the fact that there is actually tension on the mag in the holder. If you are carrying an M4 around, and you need a sidearm, I strongly recommend you have a holster that has a thumb break or a Blackhawk Serpa type of holster.
One other suggestion I have because I have a pile of these types of holsters is the Uncle Mikes hip holsters. I have a few that have a magazine holster right in front of the gun holster which is a nice way to carry a backup mag, you always know where it is in relation to the gun, and if you are carrying a lot of gear, it’s nice to keep the spare mag next to the spare gun. I have some of Uncle Mikes holsters that are leg holsters, hip holsters and should holsters. There have been a few modifications they have made to the shoulder holsters which in my opinion were an improvement. The magazine holders are now on the opposite side of the holster and you don’t have to carry them separately.