RockSlide Balltip Brass Guitar- and Lapsteel-Slide medium size: 17.5mm x 57mm. Balltip Rock Slides have two notable features compared to an original Rock Slide. First off is the very obvious ball nose which makes this a combo slide for use on a variety of other instruments like Dobro or Lap Steel. Secondly, the slide has 2 playing surfaces so one can either take advantage of the knuckle cut, or use the long side of the slides for greater string coverage.