Other Carbon Barrels are good as well, such as Shadow, or as I use a "Stiffi", These I think are also very good brands, > .685 is a good bore. As for upgrades , well there are a few, but I`ve found that Stock as you have, is good? No need to spend anything other than the Barrel, Use Only a Duracell "Ultra" Battery for a Good response from the gun, there are Plenty of 9v batteries out there but the "ultra" gives 12 x times the power of other Alkaline batteries, in fact Duracell are doing a push to the public stating this fact, This will make your Marker Work to it`s full potential, instead of you trying to work It.