Inert / Chalk balls, for training purposes. These explode upon impact with a solid surface, release a puff of chalk around an epicentrum of impact. This chalk is non-toxic and non-irritant, making it safe for target practice, as the coloured chalk will indicate areas of impact clearly. Unlike nylons and ceramics, these do not riccochet at all.
These are therefore, backyard friendly, though just be wary of the noise if shooting these with an HDR50 in your backyard.