You need to fill a "registered baggage" form at the check in desk and declare at the time of check in that you are carrying a gun in your baggage. The check in staff will get you to fill a form.

All airlines charge a fee for carrying firearms and they will charge you the same fee for an air gun too. The charges differ from airline to airline and if you have a connecting flight within India then you would also need to pay them the handling charges.

You will need to go through the red channel when going through customs and pay the duty on the item because it is not exempt. The price on the bill wont matter and nor would the item being old be of any relevance. You will be charged duty as per their own list and whatever price they have for your gun in their own list.

Having said all the above, You can buy an air gun in India at reasonable prices. If it is some specialized gun that you don't mind spending $100-150 + customs duty on top of what you have paid for the gun already then go for it.

Indian Customs Policies

Importing Firearms in India

The import of firearms in India is strictly prohibited, unless you fit the three requirements detailed below, taken from this site:

Importing Air Guns/Pistols

Air guns and air pistols are not firearms, per se, and therefore are subject to a different regulation. In particular, the import policy as defined by the Indian Foreign Trade Policy, the import of such goods follows a restricted policy. This means that there are conditions you have to meet in order to be able to import air guns/pistols.

Quoting from the ITC Import Policy, Chapter 93: ARMS AND AMMUNITION; PARTS AND ACCESSORIES THEREOF:

However, import of 0.177 bore air guns and air pistols will be free for shooters registered with Rifle Clubs or District/State/National Rifles Association. Free import of 0.177 bore air guns and air pistols will also be allowed to National Rifle Association of India (NRAI) for supplying the same to the eligible State Rifle Association / Clubs and to the eligible shooters. NRAI shall be required to keep appropriate account of the imported weapons.

So the point is, you can import air guns/pistols of 0.177 bore only if you are a member of an Indian Rifle Club or Association. In any case, you will have to fill in the relevant paperwork.

Moreover, the Mumbai Airport Customs web page is even more precise:

• Import of Arms, Air Guns and Air Pistols:

  • Import of 0.177 bore Air Guns and Air Pistols (which are otherwise Restricted as per Foreign Trade Policy) is allowed to shooters registered with Rifle Clubs or District/State/National Rifle Association and to National Rifle Association of India (NRAI) for supplying the same to the eligible State Rifle Association / Clubs and eligible shooters subject to the condition that – the purchase/user of these items has a valid “user licence” from the competent local (Indian) authority under the provisions of existing Arms Act, 1959.

  • Import of arms of permissible bore is permitted to “renowned shooters”* for their own use against an import licence issued by DGFT and a valid user licence from the competent local (Indian) authority under the provisions of existing Arms Act, 1959.

*“Renowned Shooter” means a person who has participated in a National Shooting Championship in an Open Men’s Event or Open Women’s Event or Open Civilian’s Event whether through Qualifying Tournament or Wild Card Entry conducted in accordance with the rules of the International Shooting Union, and has attained the Minimum Qualifying Score prescribed by the National Rifle Association.

Air Guns/Pistols in your Carry-On Luggage

For completeness' sake please note that TSA regulations clearly state that air guns/pistols are not allowed in carry-on luggage. They have to be checked-in, which is what you correctly mentioned you would do in your question:

I think it is safe to assume that other security handlers across the world will implement similar policies. You could always check the individual regulations for yourself.