AT44, as BT65 is too heavy.
You refer to a problem of muck. Unfortunately air rifles lack the design of many other classes of weapon and therefore suffer from jams as a result of dirt, water, and my worst problem.....fine sand and dust that gets blown into magazines, bolt mechanism, trigger mechanism and barrel....then there is salt. I often have to dismantle and clean an air rifle after every trip to the beach or estuary area. If using a .22 I usually end up just taking a modified QB78; cheap and therefore little lost from damage/corrosion. I know of one gamekeeper who often has a QB78 with 4x32 scope literally knocking around in his pickup, and have heard of others who do the same.