RockSlide Nickel plated brass Guitar Slide medium size: 19.5mm x 58mm. Medium Rock Slides have perfect weight for most styles, heavy enough for long sustain on acoustic guitars, but light enough to easily control for fast electric playing. Nickel plated-brass Rock Slides are a great combination of tone and string feel. They are slippery like glass, but have the weight and long sustain of brass. Unlike the polished brass slides, they remain slippery for a very long time.