There are several ways of making an Air rifle perform better, but most are costly. You could have an air rifle expert "TUNE" it for you. That is have the spring cylinder polishes and the spring either replaced or have it trimmed. The trigger is also another popular item to replaced. All of these can cost as much as the rifle is worth. To locate someone to "Tune" the rifle for you, try the web under "Air gun repair' or "Airgun tuning." One simple way to increase the FPS of an airgun is to put a flat thin sponge in the bottom of your pellet tin, then place a few drops of "Crosman Pellgun Oil' in the sponge. Place your pellets back in the tin and roll them around in the oily sponge. this will lubricate the pellets and they will lubricate the barrel. You will also get a faster FPS and you will notice the difference. Never use regular oil it can damage the seals. Always use Crosman Pellgun oil or RWS pellet gun oil.
Not designed for this type of modificaiton.
First, it is rifle, not riffle. Second, no, or we would shoot tanks with a rifle. Cannon are more powerful. Finally, depends on the rifle. I have a .22 rifle and a .44 magnum pistol. The pistol is far more powerful than the rifle. I also have a .22 pistol, and a 30-06 rifle. The rifle is far more powerful than the pistol (in THAT example).
270
30-06 is much more powerful.
Yes. Pump, Spring and PCP (PreCharged Pneumatic) are all more powerful than a C02 system
Yes
Yes it can but getting the current rifle "Tuned" by an expert will get you better results. Tuning will make the spring and trigger perform better.
No
If you mean .17 HMR and .22 LR, yes.
Depends on the load
The .243. It's basically a necked down .308 Winchester.
Whether or not an air rifle is more powerful than a cross bow depends on the air rifle or cross bow. Generally a hunter's cross bow is more powerful than a child's bb gun. However, during the Napoleonic Wars, the Austrians had an extremely powerful repeating air rifle. It could shoot several hundred meters. Austrian solders armed with them could shoot Napoleon's solders from such a great distance that Napoleon's solders could do nothing about it. Eventually they ran out of bullets.