Progressives are good barrels, but the best IMO, are CP 1 peices, A.K.A. Classics. Very accurate and cheap. Around $35 to $40. That "thing" under the barrel is a LPC, Low Pressure Chamber. It keeps extra air to shoot, that way it is supposed to help more air go though, there for you can turn your volicty down. And you can get nore shots per fill of your tank.
As for an elbow, that plactic one you get if fine,
http://www.chipleymachine.com/produc...eednecks_l.gif
You can get a locking feed neck, but you really don't need it. What will help w/ the electro marker is a motarized hopper. The elbow won't make much differnece. Like the 12v revvy or Empire Reloader(#1 or #2)
As to be more accurate, just practice shooting objects, and try a new barrel.
(Oh yea, the "expancion chamber" is actually the grip in the front of the gun.)