Hi,
No karan u have to complete graduation for this
You have several options:  (after completing graduation)

Join ordinance factory as an engineering graduate. UPSC conduct examinations for JWM (junior works manager) for various streams (mechanical, chemical, electrical etc.)
Join DRDO. Various DRDO labs works on different weapon platforms, they are highly specialised. As an engineering or science graduate.
Join MES (Military engineering services). At lower level they focus on operation and maintenance.
Join Defence college (DIAT Pune). PG courses are available here.
Well Weapon engineering is a highly specialised area of study which involves various streams of engineering in integrated manner. Hence in India prer equisite is engineering in any relevant branch and then opt for ordinance factory, DRDO, MES or DIAT etc.

All the best!