It depends on the model. Some manufactures will sell you one or replace it if it is under warranty. Or check the web for "air gun repair", there are shops that make a living repairing airguns.
You can find one from Crosman. They still have parts for this model. See the 2 links below.
Yes it can but it may also damage the gun or rifle if it is too strong. If you are talking about an Air Soft gun, I would talk with an Air soft shop first. If you are talking about a BB/177 airgun, you can find information on the web. In fact www.pyramydair.com offers an upgrade kit for one of their rifles. It used an air piston instead of a spring. Also NEVER "Dry Fire" an air rifle. It slams the piston against the end of the tube without any resistance and will eventually destroy the tube. There are several air gun shops that will "Tune" and air rifle buy polishing the chamber and tweaking the spring (Remove any burrs). Some even offer better trigger assemblies. But you have to weigh the cost of the original rifle and the cost of an upgrade.
depends on your definition of "smallest" depends on your definition of "smallest"
to many deaths from the use of them
Which piston? what's wrong. If it's a piston in the engine, you'll need a major rebuild. If it's a master cylinder or wheel cylinder piston, just replace the cylinder, it's not worth the trouble.
It depends on the Make and Model of the rifle. If it's a Crosman you can find drawings on their web page (see the link below)However I suggeat that you locate a airgun repair station on the web. Look under Air Gun or Airgun repair to find someone close to you. If you are handy with airguns you might be able to replace the o-rings or seals on the airgun yourself but I suggest that you give it to a repair shop and pay them to do it.
Buy a small universal piston retractor from an auto supply shop (square in shape with strange different shapes coming out of it) find the correct side of the tool, the one that best fits the grooves on the piston. Attach the piston retractor tool to your ratchet Rotate the piston back into the caliper as you would with a bolt or screw.
You have to find a repair center.
In a hydraulic system, the force on the applying piston is multiplied by the reciprocal of the area of the piston, to find pressure which is multiplied by the piston area.
The reverse of how you took it apart. I would look on the web to try to find drawings of how to assemble the BB gun. Try looking at the company that made the BB gun first, some offer drawings of the gun and how to assemble it. www.Crosman.com offer this for most of their guns. OR you can take it to an Airgun repair shop to heve it re-assembled. Check "Airgun repair" on the Net and find a shop close to you.