The most powerful .25 Cal would fall into the PCP (PreCharged Pneumatic) class of air rifles like the Marauder or Condor 25 cal. PCP's require a special compressor, Hand Pump or Scuba tank to charge them to 3,000 PSI. To give you an example a C02 Powerlett holds only 800 PSI.
However there are even more air rifles that are larger calibers like the Sam Yang 50 caliber that are also PCP class air rifles.
Some of the air gun companies are starting to make less powerful .25 caliber "Springer" air rifles like the Hatsan model 95 SAS that is very well made and cost around $150. PCP class air rifles usually cost around $300 and up, PLUS the charging system that can add another $100 to $300 to the final price.
Both Shinsung and Sam Yang make a 50 caliber air rifle. The largest caliber Weinrauch makes is a .25 caliber.
At present the Sam Yang 50 Caliber "Dragon Claw". However this year 2014, other companies are starting to make 50 caliber air rifles. These rifles are capable of taking down deer.
No
A .177 caliber air rifle has very little recoil.
air rifle
If you are asking "WHAT" not "Where" is the most powerful under lever .177 air rifle. Then I suggest the "Hatsan Torpedo 150 air rifle." I comes in at around 1250 FPS where as most of the other under levers fire at around 950 to 1000 FPS. However Under levers are not the most powerful air rifles. The PCP (PreCharged Pneumatic) Class of air rifles are the most powerful air rifles. An "Air Force Condor and the Evanix Rainstorm" fires a .177 cal pellet at 1300 FPS.
I think you have to identify your definition of power. Do you mean speed or knockdown power? A .177 caliber pellet will travel faster than a .22 caliber pellet but a .22 pellet will have more knockdown power. With 200 GDP ($324) you should be able to find what you are looking for. I suggest you look at the web site below and read the owner reviews. Then you will have more information about your selection.( See the related link below)If this rifle is avalable in the UK then this one has the most power I've seen in an air rifle that is not a PCP class rifle. Its the "Benjamin Trail NP XL 725" its a 25 caliber Nitro piston air rifle.
The model 100 is a .177 caliber air rifle. The model 101 is a .22 Caliber rifle. Basically the same air rifle just different calibers.
The most powerful air pistol would be a Air Force Talon-p 25 Caliber. This is a PCP class air pistol. The air tank uses 3000 PSI to fire the pistol.
Gamo Viper Air Shotgun it shoots a .22 cal pellet but also converts to a special Shotshell see: see the related link
It all depends on the powerplant, but generally 2 to 4 hundred yards. The most powerful systems PCP (PreCharged Pneumatics) have a much greater range. These are not as common as a regular air rifle because of their cost $300 to $3,000 but they are the most powerful of all air guns and rifles. PCP require a special charging system like a Scuba tank to charge them to 3000 PSI and are capable of firring a 50 caliber slug.
Generally the .177 caliber universal pellet or BB is the most common ammunition for an air rifle. However other pellets are available for larger bore air rifles. These include. .20, .22, .25, .30, 9MM, 45 cal, 357 cal and finally .50 caliber.