Asymetric Technologies has come up with a clever upgrade for body armor to address the problems with repeatable shouldering of an AR-15 style weapon. The ASP uses rare earth magnets in a small pouch that is attached to the shoulder straps of body armor. Then there is a butt pad imbedded with rare earth magnets on the stock of a standard issue M4 stock. When you shoulder the weapon, the magnets stick to each other. They stress on the fact that you can have a repeatable firing position even under stress or if you are wounded. At the moment there does not seem to be a link to purchase this on their website.