Refer to Section 2 (1) of the Arms Act 1959:-
(c) “arms” means articles of any description designed or adapted as weapons for offence or defence, and includes firearms, sharp edged and other deadly weapons, and parts of, and machinery for manufacturing, arms, but does not include articles designed solely for domestic or agricultural uses such as a lathi or an ordinary walking stick and weapons incapable of being used otherwise than as toys or of being converted into serviceable weapons;
and
(e) “firearms” means arms of any description designed or adapted to discharge a projectile or projectiles of any kind by the action of any explosive or other forms of energy, and includes ---
i. artillery, hand-grenades, riot-pistols or weapons of any kind designed or adapted for the discharge of any noxious liquid, gas or other such things,
ii. accessories for any such firearm designed or adapted to diminish the noise or flash caused by the firing thereof,
iii. parts of, and machinery for manufacturing, firearms, and
iv. carriages, platforms and appliances for mounting, transporting and serving artillery;
So, as per the Arms Act all airguns fall within the definition of firearms, in fact the definition is so broad that anything which can discharge a projectile is a firearm.
However, if we look at the definition of Arms, only those articles which are designed or adapted as weapons for offence or defence would fall under the definition of Arms.
Now, the Government of India has excluded certain types of airguns from the purview of all sections of the Arms Act (via notification No. G.S.R 991 dated 13-7-1962 see
viewtopic.php?f=4&t=2335#p50363 ) . So any airguns which satisfy the conditions laid out in this notification would be exempted from the Arms Act and by extension any licensing requirement.
Furthermore, as per the Arms Rules 2016, .177 calibre airguns are exempt from licensing (i.e. Section 3 of the Act) if they produce a muzzle energy of 20 Joules or less (15 ft lbs). Though, this does not exempt them from other sections of the Arms Act.
In conclusion, to answer your question.
1) If the gun in question is an airgun (a specific sub-set within the definition of firearms per Arms Act 1959), and:-
a) satisfies the conditions laid out in notification No. G.S.R 991 dated 13-7-1962 it would be exempted from all sections of the Arms Act 1959.
b) satisfies the conditions laid out in Arms Rules 2016 but DOES NOT satisfy the conditions laid out in notification No. G.S.R 991 dated 13-7-1962, it would be exempted from the requirement of an Arms License, but other sections of the Arms Act would apply.
c) satisfies neither the conditions laid out in notification No. G.S.R 991 dated 13-7-1962 NOR the conditions laid out in Arms Rules 2016, it would be treated as any other firearm and all requirements (and penalties) would be the same as for a firearm
2) If the gun in question is a firearm, all requirements (and penalties) would be the same as for any firearm irrespective of whether it has a .177 calibre (or any other calibre) and produces a muzzle energy below those as specified for airguns under Arms Rules 2016.