From AMLopedia
I dumped all the ammo in your lawn was a quote from Marcus back in 1998. The three were pretending that they were army men and that rocks were "ammo." As they would run around Adam's lawn, they would throw the rocks at "enemies." However, Adam's mom got upset with all the rocks being thrown in the lawn and told Adam to stop it. They continued playing the game, but whenever a rock would land in the lawn, Adam would say "Don't throw rocks in the lawn!" For some reason, Marcus got upset at Adam and while he was leaving, he threw all of his rocks in the lawn and mumbled, "I just dumped all the ammo in your lawn!" and ran off.