Hi mate, just in case..
yes you can use pellets but i hear that the skirts can damage the seal at the end of the cartridge, even if you put them past the seal the skirt is still passing the seal which ''can/may'' cause the damage.. Your club should let you use Gamo lead balls, but with this just make sure your change the co2 capsule before the power gets to low or the BB's can get stuck in your barrel, the gamo 4.5mm lead balls are 0.1 - 0.2mm bigger then standard 4.5mm steel BB's which are in fact 4.3-4.4mm..
Or you could see if your club would let you use H&N Copper coated lead BB's which are the standard size and will not run the risk of getting suck in yoyr barrel when the power is low, but they are not cheap.. i personally think thay are very expensive..
ATB, Marc..
Some of my favourite guns in. You collection, Umarex Beretta 92fs - Colt Custom 1911 - Walther Lever Action - TM Hi-Capa Custom Build Project - Colt SAA .45 & many more..