Don't underestimate the differences little things can make!!! Try it with out the for-end, its only a 20, just shoot it. Hold the receiver, hands off the barrel all together. I am well aware its a shotgun, but don't forget, its a SLUG gun. Its being shot and handled just as if it where a rifle. I gotta say, for-ends and the firearms latch, account for the lions share of the problems associated with accuracy problems.
Snapping the action closed concisely and consistently does make a difference. The trigger @ 4.25# isn't bad as long as its clean. Be sure the gun suprises you when kit goes off, with every shot. Its very easy to flinch with recoil or with anticipation. Even with the lead shed. Your mention the need of the lead sled along with your reminding me its a shotgun kinda says you may be "worried" about recoil.
Please do not take this as a slight, it certainly is not. Its just an assessment based on what I am reading here. This CAN help you if you choose to head the advice. Most everything I am offering here is also in the FAQ's.
I also have the USH, both in a 12 and a 20, the 12 is more accurate, but the 20 is everybit as accurate as I need it to be.
Good luck,
CW