1. Once you use an entire clip on an enemy, and still don't get the kill, you have to reload your primary before switching to your secondary. This means 1 out of 1,000 times you will end up getting the kill.
2. Also after using an entire clip on an enemy an not getting the kill, you reload and then if you try to continue with your primary weapon you will lag for 1 second before you can fire or aim down your sights, while the enemy does not lag and kills you.
These are very common problems and they need to be fixed.