The 586 L-Comp is on Smith and Wesson's "L frame." The 66 F-Comp is on the "K frame" (so is the similar 19-7 K-Comp). L frame guns are larger and heavier. L frames have a beefier cylinder, making the gun wider than a K frame. This makes the L frame a bit tougher to conceal. However, L frames are generally softer shooting (with the extra weight) and can theoretically take more of the high velocity/low grain weight .357 magnums without issue.