I'd put up some steel. wood gets chewed up pretty quick. A free swinging steel target with a wood frame to catch the splatter should work fine as long as you have adequate ventilation.
Railroad ties would work too, as long as you have a steady supply so you can change them out on a regular basis.
Also, what's behind your garage? If you're in a suburban area i'd think any setup would be a bad idea, and probably illegal.
si vis pacem para bellum