Looking at the weapon's list, it does list 2 handed weapons as W:3 - but while it's 2 handed I don't think a 20lb sledge is designed as weapon. I just have a hard time seeing a 20 lbs sledge being better at hurting people than a sword or being up there with a shotgun.
Giving it some thought, calling the 20 lbs sledge hammer W:2 could reflect how it isn't balanced for use as a weapon - incorporating the improvised nature without adding aspects. Yes, there were warhammers, but they were designed to be used as weapons (with attention paid to balance). A 2 handed warhammer - that I could see as W:3.
Richard
And a baseball bat really isn't
quite so damaging as a good one-handed sword, but they have the same rating.
The system just isn't so granular as that.
A 20lb sledge, though, is
dramatically more damaging than a baseball bat, I assure you, and as such, should have a higher rating.