WARNING: If you mod your blowgun following this tutorial, do not blame me if it leaks. I have done this several times and have never got a leak. Though at 400PSI, the trigger is a bit harder to pull.

Oh and you can choose to drill the 'pilot' (1/8" thread port) port hole bigger like you do with the 'chamber' (1/4" thread port) side, but I choose not to so I don't damage the threads where the blowgun valve threads in to.
TIP: Make sure to clean and sand all holes you have drilled before reassemblying the valve, if you drill a hole and not clean it up, the o-rings from the blowgun valve may get damaged from stray pieces of metal.
Edit: In true Spudfiles fashion, the order of the images has been screwed up. Shouldn't be too hard to understand what's going on though.