The recomendation I got from Air Force for thier PCP's was to shoot lubed pellets.
A few squirts of break free CLP or RemOil in the tin of pellets, I just give a 1-2 second spray right on the pellets, with a small sponge on the bottom to absorb excess, a few rolls of the tin every now and then, to get a few "wet spots" on the pellets, is all that is needed. The idea is not to saturate the barrel with oil with every shot, but to get the pellets lightly lubed. This way you actually clean, lube, and shoot all at the same time evertime.
Works like a charm, no rust, very accurate, no problem.
I would stay away from the brushes and solvent as air rifle barrels use a softer steel, some use brass barrels. Those shoot through felt, wad, pad, things can hurt certain guns, something to due with back pressure, when firing the light weight projectiles, lubed pellets do the same thing only better.