The tac body uses the same pin as all the mags. The pin length should be 0.750"
There are a few things you can do to check your setup.
First, turn on the safety and gas up the marker. Then hold the trigger against the safety. You should have a tiny credit card gap between the back of the trigger and the trigger rod. Too much gap and the trigger won't push the sear far enough to release the bolt. If the trigger is touching the trigger rod, then the on-off won't open to charge the front chamber.
Is your rail bushing in place? Without it, you won't get proper valve to body-rail alignment.
Make sure the bolt moves freely in the body. Install the valve without the bolt spring installed. Don't air up the marker, but hold the trigger and tilt the marker forward and back to see if the bolt moves back and forth in the breach.
Next, install the valve without the bolt or spring. Air up the valve while holding the trigger. Gently release the trigger and see if the valve vents air out the front. If it does, then you know the on-off is opening when you release the trigger. You can control the leak with the trigger, so you won't lose much air.
Except for the Automag in front, its usually the man behind the equipment that counts.