S3-Oski, yes, you can have any calibre you like as long as you have a good reason for it, target shooting is obviously the main one. Search online for your nearest shooting club or contact the NRA for details. Its only handguns and semi auto's larger than .22 calibre that are banned.
But failing that, there isn't half some fun stuff you can still get.
1 Wheelonly. Well done, keep at it. Its the NRA regs for shooting on military ranges that has changed, probabtionary memberships have always been around. As for the GSG5, great fun. I put around 10000 rounds through mine before it broke. The bolt carriage is weak and apparently they are re designing it to get over the problem. They are ammo sensitive and need high velocity stuff, and don't touch the mag when you are firing it or it jams. They need a bit of care while shooting but they are a good fun gun.
Are you going to shoot the outdoor ranges ? I might see you one day at Kingsbury.