When you bob the valve on a phantom you end up with a really big space left over. Thankfully the threaded portion of a gauge fits that area perfectly.
Bobbing the valve is just a matter of cutting off the back of the valve body so you can use less CO2. The sole purpose of the valve is to hold air and it doesn't need to hold all that much. Cutting an inch off the valve increases efficiency (about 1 to 2 shots per 12 gram). Like Terry said above the gauge it self is mostly for looks. CO2 isn't like a HPA tank. You can look at the gauge on a HPA tank and figure out how much you have left. CO2 will remain at 800 or so PSI until your second to last shot, then it will drop to under 600PSI or so and your next ball will fall out of the barrel. I have two phantoms with bobbed valves and I'm pretty sure that one of them is broken, I really wouldn't know I hardly ever look at them.
Most people put on 1200PSI gauges because phantoms are typically set up to run at 800 - 850psi and there isn't much of a selection between low pressure valves and 1200psi.
Here's a stock valve:
Same gun with the valve bobbed and gauged:
What does a full size valve with a gauge do? Absolutely nothing but cause people to ask questions.
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