Terry,this is the annoying thing about the UK limit.its any pellet and as we know ,no 2 rifles can be the same. They say bis mags are the go to for high power,not in my rifles , some might find Hades produce most power, I have had 8.44s produce way more power than heavies in my vixen.so almost impossible to know you have tested the mist efficient pellet.only way for sure to stat legal is finding the pellet you have that makes most power and tune to 10.5 or 11 max for the buffer. Do you chrono in the hot summer days? That can add more power,so don't test in winter, and a slug that makes over 12 is still illegal ,some think slugs don't get used in a official test,they can. If it's a cheap rifle don't worry so much,if you have over 1k in a rifle,stay at 10.5 lb max.dont forget if guns get confiscated you can't even remove the scope,most times our scopes are as much as the rifle.its a mine field