The Claymore’s biggest problem is that it requires a reload after every shot, and that reload time is crazy long! So long in fact that most players who run this gun in their loadout use an exploit in which you can actually cancel the animation about halfway through. The gun will blow away anything in its way, but it also has some considerable drawbacks as well, which keep it from that top spot on the list.
Many people consider the Claymore to be the best shotgun in the game because of how crazy powerful it is, and boy is it that! The M-300 Claymore deals more damage than even the M-98 Widow sniper rifle from up close.