Yep, Elkein is right, one foot is more than enough for .22 rimfire... I use a washed white sand available at any home supply center for $3 a bag... I have a heavy steel military ammo can... At first I used about a foot of dirt, but it dryed out and caused a dust cloud when fired into... Changed to 1.5 bags of washed white store bought sand, I covered the sand 100% with several layers of old bath towels... The towels reduce sand dust by 98%... I shoot indoors through towels into sand... Most spent .22 rimfire bullets are to be found less than 2 inches down... I checked to see if the towels might catch on fire or smother, they never have, its worked safely for years... I know fired bullets are hitting spent lead in the sand and creating large globs of lead, but I never clean it, its safe, someday finding a legal way to dispose of this lead/sand mess is up to my heirs...