If you're new to the Handi-Rifle, the guys over at Greybeard's Outdoors have put together a Handi Rifle basic FAQ thread. In it you'll find a wealth of information on the Handi-Rifle.
For example, one of the quick tips for accurizing the rifle is to put an O-ring or neoprene washer between the forearm and the barrel, over the mounting stud. This tends to isolate vibrations and helps the rifle shoot better. There is also a great list of scope mounting parts and part numbers to make your search easier.
I was at Lowe's today and picked up a pack of O-rings. Installing one takes about two minutes and that's a quick, easy way to tighten your groups. Some rifles need it, some don't. Now, I've got O-rings enough for ten rifles. The rate I'm going I might need all ten of them.