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
Each one has its own unique characteristics
18 years.
No
no
no.
Depends on where you are.
yes
Do you mean an air Rifle? NO, Not in the United States.