Well it’s not a toy is it? I bet that it has “this is not a toy”’ on the side
However “airsoft toy” is a fair one
You must declare if it has an air system, (which can technically include parts of airsoft guns) or batteries etc - and ensure it’s properly marked, packaged and using an appropriate method
If it’s just a RIF with no “restricted” elements, and you’ve checked it’s ok with the carrier then just make sure there is a fair declaration for when it passes through an X-ray scanner along the way