The off shore large bore air rifles are okay.
When I began to look for one several years ago, I decided that I wanted a US made PCP large bore.
Dennis Quackenbush builds some very nice PCP airguns, but his back list is years out. I then discovered a fellow in Louisiana that built large bore PCP air rifles, Jack Haley at Southern Airguns.
I own a Jack Haley (RIP) .457 cal PCP air rifle. Jack custom built his guns to order. Shortly after getting my .457 cal rifle, he introduced a .58 cal air rifle. At a 100 yds I am getting 3" five shot groups with it using 340 gr pellets. With a 3500 psi charge it will provide 5 - 6 accurate, lethal shots.
I've shot 'pellets' ranging from 340 - 505 grain HP's and .457" lead ball out of it.
The guys in the US hunt wild pigs, deer using these types of air rifles.
Here's a couple of Youtube videos of a JH being shot at distances one wouldn't think an airgun could reach out to.
180 - 200 yards
http://www.youtube.com/watch?v=VYFzhiXrStY
A .457 JH compared to a .25 Theoben
http://www.youtube.com/watch?v=3PP6NNST_7o
Here is a short video of one of my sons shooting my JH. That's a 505 gr HP 'pellet' being loaded in at the start of the video!
http://s277.photobucket.com/user/blackacres54/media/Jack Haley Video/VID00004-20101104-1751.mp4.html
From 30 - 40 yards away, you would think you just heard a air powered frame nailer discharge, it does not have the typical high velocity crack one would associate to a rifle discharge.
She's a pretty cool beast to take out once in a while to play with.