I grew up in the early 60’s on Lacombe Avenue in the Castle Hill Avenue projects in the Bronx. We were privilaged enough to have a skully (skelly) board neatly painted in yellow on the playground area outside. We experimented with a myriad of bottle cap types and fillers. The best “shooters” were the little white (soft) plastic prescription bottle tops. (This pre-dates child-proof caps.) Since the precription bottle caps were much lighter than metal bottle caps, we would weight each of them down with a penny and then fill the cap up to the top with candle or crayon wax. When the wax dryed, we would rub the top of the cap on the ground until it was super-smooth. The plastic precription bottle caps were the best shooters. When you would shoot one, it would ride smoothly along the ground, travel farther than a metal bottle cap, and, with the weight of the penny, it had enough weight to smash a competitor’s cap a mile away. I moved to Long Island, NY when I was 12 and miss playing skully.