BARREL FIRST - most of the time. The stock a-5 barrel is not that bad though!
Ex-chamber do NOTHING on HPA, and very little on CO2. They are a waste of money. Trust me, I've wasted that money. A regulaor, and a good one at that, will increase your consistancy far greater than a ex-chamber could ever hope to. All a exchamber can do is provide a few "wide" spots in the gas flow to allow liquid co2 to try to become a gas. It will do nothing to the incoming pressure of that gas however. A regulator will limit in incoming pressure to a set pressure ( well, with in reason. the best regs are only rated to 100/1 - which means for every 100 psi of incomming change, the regs output pressure will ony go up 1 psi )

Internal mods (starfire, power tubes etc) do nothing to help the marker work, infact some actualy hurt the preformance of the marker. I'll have to find the thread on that....

HPA is a ton of cash, but you can't start ripping 20+bps on co2. It just can't handle the speed. I have 1 HPA , and 3 20oz co2, and use all of them on the same marker, I just need to limit my speed on the co2. Thats all.