LEGAL: Use subsonic .22 ammo. The old nickel-plated RWS stuff is my favorite. Zero for 75 feet and you should be fine (note that the bullet drop in a larger subsonic hollow-point round is dramatically different from a standard or high velocity round).
NOT-SO-LEGAL: (for informational purposes only). Once could theoretically:
*Core out a potato and stick on the end of your rifle barrel.
*Take a two liter soda bottle, cut a hole in the bottom, and fill the sides of the two liter bottle with shaving cream. Attach contraption to barrel.
Note that these theorietical field-expedient devices, attached to your barrel, will affect barrel harmonics and thus bullet placement.
In any instance, aim for the head.
-WW