[dmitril]

Duster gas is just a lower pressure gas you can use for airsoft. My advice is avoid upgrades unless you know you won't "feel lazy" midway when the money and time is spent and nothing is working yet.
Also, if you want a dmr gbb stay away from variable valves. You can adjust the valve timing very accurately(and therefore the fps) by using shims. You have to google this up as I don't remember where I saw a guy doing this.