Re: shoulder firing with a pack on
OK, from the standing how are you shooting? Are you squared up on the target (both shoulders facing the target) or are your shoulders in line with your shot? If you're not squaring up on it, you'll definitely have problems. It would force you to place your buttstock up onto your deltoid and not in the pocket.
Don't shoot like you're on a KD range shooting offhand. You need more of a "tactical" stance. Plant your feet shoulder width apart, knees slightly bent and torso slightly leaning forward, and place the buttstock more into the center of your chest rather than the pocket of your shoulder, more toe than heel making contact on your clavical. You should still be able to keep your head erect so long as you keep the stock high enough. If you're a shotgunner this stance should be familiar to you.
You'll have to master your trigger control as this is a less stable position to shoot from, but a hasty sling can help with that. Be ready to quickly but smoothly pull straight through the trigger when your sights/reticle are lined up on target, but watch out that you don't jerk the trigger. You can also use interrupted trigger control if your sights are wavering too much or you start to jerk the trigger.
ETA, Lonewolf's points are spot on too.