It will be marked on the barrel.
It fires .22 Long Rifle bullets, and leaves behind shell casings when you eject them from the cylinder after firing.
Yes, they are all high brass and considered magnum loads
When were 3inch magnum
No, it is/was not.
In a .44 Magnum revolver, yes. In a semi-automatic .44 Magnum (such as the Desert Eagle), no.
The sizes of shells it accepts will be stamped on the barrel
The Light will only accept and shoot 2 3/4" shells. The Magnum will shoot up to 3" 20ga shells.
In general, no. Unless the gun is marked for 3 inch (or even 3 1/2 inch) shells, you should not attempt to fire 3 inch Magnum shells, even if they seem to fit. However, if you have a gun made for 2 3/4 inch shells, and have 2 3/4 inch "magnum" shells, IF your gun is safe to shoot, it should be safe to shoot those "baby" magnums.
No problem finding them on line in catalogs.
No, they are different sizes.
Is it chambered for magnum shells? If not, the answer is NO.