barrel, hopper, maybe a trigger depending on what yours came with
Easy to clean
A bit on the heavy side
This gun is great, i bought it used off of a guy who took very good care of it, im not sure what parts on it were upgraded, but it came with a Long shot barrel, a Dye grip, a smaller trigger wich i dont like very much and am planning to get a new trigger+frame, and im pretty sure it has a stock timmy bolt.
Anyways, it has been awesome. When i first got it i had gravity feed. Omg that was horrible. But now i have a Halo and it has been so great.
It hasnt chopped once, the ACE is great, though it cant see black paint (who uses black paintballs anyways?) It is a bit heavy, and my friends complained about that since they use Ion's, and my Halo adds more weight, but its not that bad, go lift some weights.
The only problem i had was finding a volt that fit inside the gun. Im sure its not the gun, its these screws that come off of the . . . air port thingy. . . whatever thats called, any ways the screws that hold that on stick up inside the gun and it gets rid of some room for the battery, but im sure if shortened the screws any 9v would fit.
Anyways this gun is great!
If you are looking for a gun you can find this gun for pretty cheap from local players at fields or at your local pro shop. (i got mine for only 350$ with upgrades and a warranty from the pro shop)