-The higher the velocity the more gas per shot will be used. With the current tune I expect about 30 shots from 2 carts. Hopefully the HDD put it back to about 40-45 good shots.
-The HDD will help with co2 milage definitely the way to go in my eyes.
-To decrease gas usage, I believe a lighter striker is needed. I will be going there soon, just need to wait till I can get or use the correct tools. The trick is to lighten the striker such that velocity is not affected, and play with the striker sping at the same time. A ligther sticker will decrease the effectiveness of the HDD as it willl have less bouce. But a lighter striker often needs increased spring preload from everything I have read here. The higher the spring load the less effective the HDD is so everything is a balance between power and effiecency.
-Valve mods are really need to get top velocites, but moderate velocites can be had with the few minor tweaks mentioned above. (XP tune kit, Breach seal and barrel port opened up some, aligning all the ports for perfect smooth flow)
-Go .22 if you need power .177 won't get there as easy and is alimiting factor for effiecncy and power.
-The more effiecent the co2 usage the less the cooling effect and the more consistant the velocity of a co2 gun.
-Now the trick is to put all this together in a system that works together and is balanced. There are many experienced tuners that can do that for you. For example opening up the barrel port does not do also much if you don't probe the gun (drilling the probe out to 5/32 helps but the bold becomes a limiting factorfor constricing flow). If nothing else is done not much sense in valve mods. The idea for a tun to to do everything such that it works as a single unit and balance is needed. Open the barrel port to much and the pellts drop in and block... as real pita and not the easiest fix.
Another option if you want lots of power is HPA, I have no experience there but their are lots of peeople here that can help you.
I found this very intersting and worth a read
I don't know what effect the shorter barrel will have but I know most co2 guns can get nice improvments in velocy just going to a longer barrel. Maybe get a 830FPS tune and start cutting an 1" at a time off the barrel till it is as short as you want, or hits the min velocity you want? Don't forget a re crown will be needed etc. Might be more a job for a pro tuner or start with the Archer carbine and live with the max you can get?
I found thishttp://www.network54.com/Forum/113813/message/1214869394/my+QB+14%26quot%3B+carbine+shooting+600+and+just+under