Customization is allowed but customizations have there limits and you need special permit to do so..
But the Issue is >
1. Rifles such as AK & INSAS so does FAL have Odd mounts which do not match International stander ed ( P-rail )..
2. There are no Major foreign Optical Stores in India let it be for Commercial or Professional ( Only for Law enforcement ).
The only way is you have to make a mount for your gun and Purchase the optics from Outside India or if there some Legal agent in India..
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I hope most here understand how twisted situation is even if someone wishes to have optics for himself, let alone the expenses involve it has more to add into this..
If we had military surplus shops where IA soldiers could buy gear to customise their weapon would it not be better.
I mean giving them permission to add sights at their cost if they want is it not feasible?