No.
for a rifle it is thought to be a .50 calibre but a gun company made there own for there own sniper rifle the .408 calibre
yes, you can and need a license to own an airsoft gun in nyc.
Requires professional appraisal
18 years.
No
yes
no
Depends on where you are.
no.
Each one has its own unique characteristics
Do you mean an air Rifle? NO, Not in the United States.