AJ's Revolver is a Smith & Wesson Model 327
I noticed while watching a let's play that AJ's revolver an 8 shot revolver. I had noticed it before when playing on my own, that it looked like it had too many chambers but I was busy and quickly forgot about it. Then watching a let's play I noticed it again and decided to look into it. I worked out through stills from in games scenes that the cylinder has 8 chambers. 8 chambered revolvers aren't exactly common. After some digging and making some comparisons, I discovered that the Smith & Wesson Model 327 matches up with the in game model about as well as anything can. Here's an image I put together from in game stills. The only image I could find with a good profile view of AJ's revolver sadly has it out of focus as Louis' horrified face is what the focus of that scene is. Still, I think it's good enough to back me up. The handle is slightly different, but handle plates are kinda' like sticker decals on your laptop or window glass. They're easy to swap around and customize. The barrel on this model is also customizable, like changing the wheel size on your truck or car, so some barrels are even shorter, some are longer, but this one in the image is a good base representation of the model.
I just think it's nice that TellTale did some solid research on the revolver that AJ has and it's not a generic 6 shot revolver like you'd expect to find in a game.