The longer and tighter your barrel, the more regularly it will require cleaning. Standard barrels should be cleaned after 5,000 shots, 6.07mm barrels should be cleaned after 2,000 shots and tighter barrels more frequently still. You MUST use high quality ammunition in upgraded barrels otherwise you will risk getting jams. Jams cause stress build-ups on the gearbox and this is the commonest seed of destruction for gears. This means that the BB starts to reduce the air pressure in the barrel behind it, which will act like a slight sucking action, holding the BB back from full speed.
Standard barrels are available from most skirmish sites who service and repair AEGs.
This will increase your range and accuracy. You will find that 0.25 gr BBs are more stable in flight than their lighter cousin, though to maintain the range, HOP-Up must be set quite strong. Ultra high-grade, extremely small diameter rifled barrels are VERY rare. This counts as a five star upgrade, due to rarity and price. Designed for target shooting, or sniping, these are totally impractical for general skirmish usage.