Is the Sigma 50-500mm f/4-6.3 EX DG HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 500mm f/6.3 test image. Observe the contrast and resolution of these results.
Is the Sigma 50-500mm f/4-6.3 EX DG HSM Lens set to 500mm f/6.3 sharper than the Nikon 300mm f/4D AF-S Nikkor Lens set to 420mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.