Hi y'all. I am new to air rifles, I'm a semi auto shotgun user but due to a very short and limited hunting season here in spain on designated hunting grounds, namely october to february and only on thursdays, saturdays, sundays and public holidays, Having permission on quite a lot of private land, and in order to not attract the attention of the authorities I decided to buy an air rifle, I bought a Hatsan 6.35 calibre (.25 in old money) 24 joules breakbarrel, scope, sound moderater etc. In the UK being over the legal limit of 16.7 joules this would require a full firearms licence. Anyway my point, is there any way to 'Tune it up', example get more power from it than the 24 joules it already produces..? It does feel powerful already and can stick a solid open point slug a good inch into a builders plank from around 40 yards, but would just like a bit more from it.
 
@slydivecarl - More power will not make the rifle more useful. Shoot it in stock format and see how accurate it is. I suggest a good pellet like H&N Field Target Trophies or JSB 25 grain.

High caliber break barrels are notoriously bad for long range shooting because of the pellet dwell time in the barrel. Try a .22 or .177 for a pesting break barrel, they have much less dwell time in the barrel.
 
Yes 24 Joules is the limit in Spain. and to be fair I am quite impressed with it, as I said I am a semi auto shotgun user and for an air rifle it is impressive, and understandable why in the UK it needs to be on a firearms licence the same as any other rifle. Here I am allowed upto 6 shotguns and 6 rifles on my licence, the point of the air rifle was to not attract attention (noise) whilst roaming around the private land I have permission on. Any advice would be welcome if any of you actually hunt with an air rifle. I have found whilst target shooting with a target fixed to a thick builders plank the H&N Grizzly have much better penetration than normal slugs, much the same as a regular bullet, a good inch into the wood so I imagine they would have a serious effect on biological matter, they are hollow point and make quite a big entrance.