Air gun and gun questions.... I've always wondered......

[mojomoto]

What is the advantage of a bull barrel? Weight? Barel more rigid?
What is the limit of usable pressure in an air gun?
What is the highest pressure carried in a pcp?
Why doesnt someone make a diesel gun. (on purpose 

[RedFeather]

I can answer a couple, sort of, maybe.

Last question first.  There is currently an intentionally dieseling air gun, the Mendoza.  These have a small hole in the top of the compression tube through which you add one or two drops of light oil.  The piston has a fiber washer behind the conventional seal and this sits beneath the hole when the gun is at rest.  These guns shoot hard but inconsistently as the oil wears off.  Not a real success.  Another gun, although not really a diesel, was the Barracuda.  This gun was a modified springer that contained an ether bottle, dispensing a bit of ether as the gun was cocked.  Being highly flammable, you can imagine how volatile the resulting shot cycle was.  Again, not a good idea.  In fact, I'm not sure if a few guns had mechanical failures.

Usable pressure in an air gun is determined by the max flow rate of air as a gas.  It's something around 1650 fps.  So, depending upon bore size, projectile weight and friction, I guess the most efficient peak pressure is that required to accelerate the projectile to around that speed.  (You can search this on the Yellow.)

Bull barrels are fine on powder burners because they are stiff and heat up slowly.  There's not a lot of advantage, to my mind, on a springer.  Maybe stiffer in a shorter barrel but that's about it.  Aside from cosmetics.  A solid steel bull would be on the heavy side, so most you see are hollow shrouds or plastic over liners.

[mojomoto]

Thanks for the cool info RedFeather!