One thing that is stressed in firearms training is that you have to constantly be active and practice your firearm skills or you will most likely loose your edge. Just remembering the feel of recoil, gun grip and knowing how loud your gun will be if you don’t have hearing protection on is something that is easily lost. There have been times where I fired guns so much that I practiced training without hearing protection on open ranges and the sound of the gun really didn’t cause me to flinch, but there were times where I’ve fired guns in indoor ranges and the guy standing next to me made me flinch every time his firearm discharged because even with hearing protection, the blast was so loud in an indoor environment.
If you practice with something like an AR15 enough, you will be able know the difference in the cycling of a firearm and when the last round has gone off and the bolt has locked back, but one thing that needs to be thrown into the mix is will you know this under stress? The short answer is probable not. The Magpul pmags with the windows will give you some point of a reference if you are under stress that a simple tilting of the firearm can tell you if yo have 30% or less of your ammunition left. There’s no reason to drop a mag and have to shake it or waste a partially loaded magazine with a another fully loaded. If you’ve got 8rds of 5.56 ammuntion you aren’t out of the game and a simple magazine window is an asset.