The mosin has an internal magazine. There was a "stripper" clip in the PC version that would allow you to reload all 5 bullet at the same time but has not been implemented in 0.62/0.63 yet
Damage is based on blood, health, vital organ health and location of the hit. There is a couple good youtube videos explaining this. This is the best one I found (Part 1 & 2)
If you go into your TV settings you can change your Aspect Ratio and it will fix when you find the right ratio so you can see the whole screen. If your TV does not have this option (some TVs don't) I would drink half a container (bottle or canteen) every 15 minutes roughly but especially when you first spawn since you start at half thirst. Good luck :)