I second Proffink on the TM VSR-10 range of Springer sniper Rifles.
I bought one second hand on a forum and it had some upgrades done it but not alot.
The previous owner had already upgraded the cylinder, piston, Spring (to an M150 i think) spring guide, cylinder head, installed a zero trigger mech and a hop up arm, it shot ok-ish around 465FPS and fairly straight though shots would veer to the right often. I furthered this by zeroing the scope using a laser bore sight as the scope was well off whack, i shimmed the hop up arm to keep it still, bought a new barrel and barrel spacers so not it shooting straight but still around 465FPS, my field limit is 500FPS.
That said you must be prepared to know that Spring Rifles can be very difficult to cock back as your right arm will have to fight that spring in order to fully pull back the bolt (about 3 inches worth of bolt) and frankly after a whole mag of trying to shoot quickly your arm will ache.
That said if you find the bolt pull too much hassle like i did as its really heavy, there is a website called
http://www.Mancraft.pl obviously based in Poland, there they sell a system called the SDiK which is essentially a refit of the piston to a gas cylinder, you can buy all the accessories from them too including regulator (HPA tank compatible) and a CO2 bulb adaptor. The whole system takes less that 30 minutes to switch from spring to CO2, i managed to stow the regulator and CO2 adaptor in a cheek rest for my VSR-10 and pipe the tubing through the stock so it looks very clean. All in all expect to pay around £150 for the SDiK, regulator, piping, CO2 adapter as well as some spares and delivery.
Bolt pull can be done with one or two fingers now and only has to go back an inch, as it is regulated i can set it up for an FPS of 497 consistent shot to shot (some times even 3 or 4 shots in a row), one CO2 bulb lasts about 100 shots, i also have the ability to adapt it to HPA which i probably will when i start polarizing.