Is anyone experiencing accuracy problems with the Crosman Phantom .177 cal. air rifle?

I've tried the "iron sights", a Tasco red/green dot sight and a Tasco 4x power scope and can't seem to get under a 5" group at 15 yards. At first I thought that it had to be operator error but after going through two different types of optics plus the factory sights, three different types of pellets (including the recommended Crosman "SSP" pellets), two different shooters (USMC Scout Snipers), a bench rest and a shooting vise I keep getting the same results. In a three shot string, two holes touch and there is one "flyer" almost every time. I keep reading all these reviews about how accurate this rifle supposedly is but I'm not seeing it. There is no fouling in the barrel and the only thing that I can figure is that the air charge is inconsistent.