I suppose I could actually be helpful and answer your questions:
caseyboy wrote:Is there a definition of unaltered?
In the rulebook, no, there is no definition of unaltered.
caseyboy wrote:This is quite ambiguous.
This is true.
caseyboy wrote:If you were to tune the power plant, cut a couple of coils off the mainspring and polish up the trigger contact points and possibly tweak or replace a spring, the rifle would be altered.
This is also true, but it would not be illegal.
caseyboy wrote:Where is the line drawn?
It's drawn somewhere after the alterations you mentioned first (above) and before the alterations you mention below.
caseyboy wrote:What about replacing the stock to something that is similar to factory, but fits a little better?
This is a BIG no-no.
caseyboy wrote:How is any of this approached at the "big" matches?
There are only three "big" air rifle silhouette matches in the country (there should be more, but that is a topic for another post); all in Louisiana and Texas. They are the NRA National championships in Baton Rouge; the Southern Nationals in Winnsboro, LA (soon to be renamed) and the Texas Nationals in Houston. As you can tell, silhouette shooters are an unimaginative bunch and they name every match the "(fill in the blank) Nationals," but that too is a topic for another post.
At all of these matches, the rule of thumb for a Sporter rifle is that you can alter the parts that you cant see, but you cannot alter the parts that you can see. Basically, you can do whatever you want with the insides of the rifle as long as you don't make it shoot so fast that it damages targets; and you CAN NOT replace the stock (although you can alter the factory stock to make it fit you better as long as you don't add something like an adjustable cheek piece or a palm rest - basically you can lengthen or shorten the length of pull or add/remove weight to the stock.)
Hope that answers your questions.
Dustin