Oiling your gun on regular intervals is important. In your case, if the pin is stuck, try the following.
Put several drops of oil directly on the pin and slide channel. Remove all nails, reassemble the nose and see if the gun will dry fire against a scrap piece of wood. the gun should fire and leave a dent from the push rod in your wood if it is working.
If the gun still does not fire, then the pin may be damaged or bent. At this point the upper air chamber will have to be removed and the pin assembly removed/replaced. Rebuilding air guns is not real difficult and parts for a Bostich are readily available. You problem may be a worn or damaged O-ring in the compressor section. Rebuild kits for the upper end have got to be replaced often, especially if they have not been oiled regularly of after long periods of storage. Parts are cheap.
Some folks at the tool dept of Lowes or HD could be helpful guiding you. If you have a good lumber yard that sells Bostich tools, they often stock the parts and will be helpful in helping you fix your tool.
In my area, Bostich often have promotion displays at the vendors where they will do maintenance and install rehab kits at no charge with the purchase of a case of nails. Both of my framers, Bostich 88W's, have been rebuild at least 4 times for free during these promotions.
Take it to a shop that either does hydrostatic testing, or has an arrangement with a place that does. It will cost far less than $100, barring a major ripoff. Any place that deals in compressed gasses (welding supply or diving supply) needs to do this on a regular basis (5 years per tank), as do fire extinguisher service shops (which might actually be the place your dive shop sends a tank to for testing.)
The formal tests (in the USA) are based on Department Of Transportation regulations for portable tanks - fixed tanks are not required to be tested (at least by the DOT), but can be. Twice working pressure may be more than is correct - I'm more familiar with dive tanks, and there it may be as little as 5/4 rated pressure - failure is not "burst only"; a tank can fail the test because it permanently deforms (the tank is submerged in a tank of water while being tested - the expansion of the tank when it is filled causes a measured about of water to leave that tank. If the tank being tested does not shrink adequately after pressure is released, the tank fails testing.)
Most fixed tanks fail from internal rust due to lack of proper maintenance (draining the tank regularly.) If your tank is 40-50 years old, it might be worth simply replacing it with a new one, particularly if you have difficulty getting it tested at a reasonable price.
Best Answer
Not dangerous, but you'll eventually stop shooting nails. Chances are it's just a couple of o-rings that need replacing. Look for something like a 'cylinder rebuild kit'... should be well under 20 bucks.