Mark,
You need to have loads of a known pressure, usually 30% more than the SAAMI limit. These are sometimes called "Blue pills". They are not for sale to the general public. You can send the gun to Birmingham or Liege and have them proof it for a fee.
Make sure there is a solid wall in between the gun and the tester.
Measurements are taken with instruments that have been calibrated and certified to assure accuracy.
Here is a thread on the subject that you may find helpful.
http://forums.nitroexpress.com/showflat....=&fpart=all
