The key to shooting unsupported is strengthening the supporting muscles. Your large muscles hold the rifle up, but the small muscles around the larger muscles are the ones that give you support and balanced. The best analogy I can give is watching someone who handles large and bulky items at work. The person maynot be very big, but handles the objects with ease while you will struggle moving the same objects. The worker has toned his supporting muscles through doing the work over time.

The best way to accomplish this is through dry firing. Check the owners manual to see if dry firing is ok for your rifle, or else get some .22 snap caps. You will need a place to practice- I use my basement and put a dime sized piece of tape on a wall and stand back 25-30 feet. Triple check to make sure the gun is unloaded, also make sure there is NO ammo even in the same room as you are!

Bring the rifle up and set the stock on your shoulder so you can see through the sights without craning your neck- the butt of the stock can stick up above your shoulder- this is OK. If you are right handed, make sure you don't drop your right elbow- keep it at least parallel with the floor. Place your left elbow against your ribcage to make an open ended triangle with your support arm with one side of the triangle being your upper arm against your body. Support the rifle on your finger tips, knuckles, or 'Y' made between your thumb and index finger- don't bend your wrist if you can help it, and dont grab the rifle stock.

At this point the rifle is going to be incredibly wobbly. Take aim at the tape on the wall and slowly increase the pressure on the trigger, ideally you want the trigger to break as the sights cross the target, but this isn't going to happen as often as you like until your support muscles are built up. After the trigger breaks, you need to follow trhough- hold your position and try to imagine where the shot went- Follow through is a very important fundamental that most people don't even know about.

Lower the rifle, and rest your arms for a moment, then repeat. Its boring, so only do it a dozen or so times a day so you don't get into any bad habits- remember perfect practice makes perfect. Eventually your muscles with strengthen, and the wobblyness will settle down- it will always be a little wobbly but with good trigger control, it doesn't matter much.

If you have access to a shooting range, take a couple of soda cans to shoot at- they are reactive targets and will instantly tell you if you hade a hit or miss, and gradually work down to smaller targets such as golf balls.

Don't expect miraculous results overnight- it might take several weeks to see a good amount of improvement. Shooting offhand is not easy, is discouraging, and has a long learning curve, and doesn't give nice cloverleaf groups to hang on the wall or post pictures of on sites like this, that's why very few people practice at it or ever become proficient at this skill.

I would bet my next paycheck that you are flinching/jerking the trigger/anticipating the shot or a combination of the three. If you don't have good trigger control, your body will start reacting to the shot before it even goes off, most often pulling the gun low.