Hatsan AT44-10 25 Caliber

Bunch of good stuff (used to be) on Youtube, things like jb welding cylinder ports and the like. There are forums out there with rich details on modding them. Before doing anything, spend some time with no mods, get to know what pellet it likes, and how well it does/doesn't shoot. I had one that was a turd from day one, and the next one was brilliant. No need to polish a turd, at best you'd have a turtle.. 
If you fall in love with it, set realistic expectations. Maybe you want 30 decent shots, maybe you want to shoot elephants for 10 shots. Pick one, and develope the rifle along that goal. It wont be an fx/Edgun/Daystate, but it can hang with them for 10-15 shots, with the right setup, pellet and practise. B Walton has some amazing videos of hunting with them at incredible distances. You may not be a B Walton, at least not at first, so allow yourself time to grow.
 
Do you have the long or standard? My standard is tuned and regulated but it takes a lot of work to make the standard shoot regulated with decent power. The valve is decent enough that the gun wil perform great without a regulator. It only have a 180cc air cylinder so with a 25 cal it will not get a lot of regulated shots unless you do some modifications to the valve.
 
I had a similar experience as Oakhonor. My first pcp was an AT44 .22 short. I regulated it and did the valve mod. It has proven to be easy to work on, very reliable and extremely accurate. I then bought a Nova .25. The Nova responded well to the valve mod and gave me a really good shot string with a low extreme spread - however it's accuracy was not acceptable to me. I found a number of problems with the barrel and despite quite a bit of work I put in to trying to resolve them I had to set it aside for awhile (lest I wrap it around a tree). Yesterday I decided to give it one more chance and ordered a new barrel for it. Hope this fixes it. If it is even half as accurate as my first Hatsan, I'll be a happy camper.