in general 180 $ is the minimum for a reliable rifle.
When you say Air Rifle you are referring to a pellet rifle not a firearm. At present there are no 300 Cal air rifle being made.
The minimum age is 17 for an air-gun (rifle or pistol).
It depends on the power of the air rifle, Local laws and the location where you live.
Depends on the air rifle. England restricts certain higher powered air guns that have enough energy to be useable as a weapon.
It all depends on the country where you live
There is no legal limit for PSI for and Air Rifle. Many PCP Air Rifles Have extremely high PSI's and are charged with a scuba tank for mutable shots without a recharge. About Modified: Without taking the rifle apart and knowing what to look for it is unlikely that you will notice any change unless it is obvious from the outside.
The length of the barrel. Guns/pistols can be held by one hand. While a rifle requires two hands to properly hold it. Both are air powered.
There is no legal limit for the rifle in the USA. But you did not state where you live. So I suggest you contact Air Venturi for further information. See the link below for the owners manual and telephone number for Air Venturi in the USA.
Yes, an air rifle is not considered a firearm by Texas or federal laws.
You will have to contact the California Department of Justice for a current, legal and correct answer.
Yes- air weapons are amongst the few firearms it remains legal for a private individual to own in the UK.