Assuming you are running the stock bolt and the orings are in good shape, and the bolt choke screw is all the way open and indexed properly (the bb is not sitting out of the choke screw groove), and your valve is rotated to use the largest port, then the main thing that will determine velocity will be your tank output pressure. I'm in the process of rebuilding two VMs, I just got one going and waiting on some parts for the other one. I have everything wide open and using co2 it seems to be shooting at or above 300 (haven't put it on a chrono yet but it's definitely shooting on the hot side, which is good and it means I can dial the velocity down using the bolt adjustment, which won't affect the re-cocking action). I built a testing rig to check the output pressure of my hpa tanks vs the co2 tank I was using... the co2 tank is outputting just over 800 psi. I have two 'high pressure' Pure Energy hpa regs that are outputting 850 psi, these will work well on the VM and should get me good velocity. My other hpa tanks have 'normal' regs on them and they all run about 700 to 750 psi, so not enough to get good velocity on the VM.
As noted by above adding some volume in the form of a dummy expansion chamber or gas-through grip would probably help, you might be better off running a bottom-line setup with a volumizer in the front asa instead of running the tank directly into it. One thing to note about drilling out the valve, in order to benefit from the increased size of the valve port, you would need to also drill out the air passage in the gun above the valve and you also risk affecting the re-cocking action of the gun (more gas going up through the bolt and less gas pushing the hammer back). I will bet that if you find a hpa tank reg with output pressure at or above 850 psi you will be good to go.
As noted by above adding some volume in the form of a dummy expansion chamber or gas-through grip would probably help, you might be better off running a bottom-line setup with a volumizer in the front asa instead of running the tank directly into it. One thing to note about drilling out the valve, in order to benefit from the increased size of the valve port, you would need to also drill out the air passage in the gun above the valve and you also risk affecting the re-cocking action of the gun (more gas going up through the bolt and less gas pushing the hammer back). I will bet that if you find a hpa tank reg with output pressure at or above 850 psi you will be good to go.
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