I had one, I mucked around with it a LOT, accurising etc. Got it to the point that at 10 metres I could put an entire mag into a single hole group. Accurate enough to hit coke cans out to 40m. I didn't rate it as enough grunt to shoot any living thing and expect to kill it - Mostly around 350-400fps with 8gn pellets. They aren't really semi auto, they are a double action revolver set up - and as such the trigger pull is long and heavy, not nice at all actually. "Muzzle Brake" is just tacticool, although the purpose was effective as it held the end of the inner barrel in a somewhat fixed position compared to the factory setup which didn't hold the barrel much at all. This is the biggest mod to increase accuracy - as well as crowning the end of the barrel and cleaning it up.
Also, you totally shouldn't do this, but a CO2 cannister dropped in a bucket of HOT water for a couple of minutes will give you a first round velocity of 1775fps approximately - which was good for a laugh... once.....