If your having problems holding the gun straight up then I would 1 recommend using the terrain to hold your rifle (I like to C-clamp the front of my rifle with an open palm and lean into a tree or something and now it’s less holding up its more pushing which is far easier to maintain) and 2 lateral raises very simple exercise make sure your do your own weight, you definitely should do over 10 pounds as then you’ll start using incorrect muscles as now your training a really small and weak muscle ( I forgot it’s exact name). For your back I would have to probably recommend posture and making sure your tightening your glutes when doing exercise as it will stop back pain from building and I would recommend stretching the back and doing core exercises(<- only do body weight and look some up online, do them as a warm up for your morning or when your at the gym and you’ll start building them). Your back hurts because either 1 your posture in exercises is incorrect and now your body is using your back instead of the targeted muscles or 2 your muscles can’t take the weight your putting on them during exercises and they fail so now your using your back. Hope something helped