Sounds like you need upper body toning. Cardio isn't that. Upper body toning is about your back and shoulders more than your arms, but if you have spaghetti arms and weak shoulders, go lift some weights. Doesn't have to be really heavy weights though.
There are techniques for shooting off hand that can vary according to the rifle too. Operative word being 'can'. Starting with not having your arms out like you're trying to fly.
An M305(that is not an M-14) with the synthetic stock is held differently than one with a wood stock. It's about the weight and balance. The wide front sight doesn't help hitting an 8" target either. A scope will let you see how much you're wavering on and off target, but will also alter the balance.
