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Dear Vivek!
To do weapon engineering you must b graduate as this is a post graduate courses. Do your B.Tech in Aerospace & aeronautics engineering into the specialization of 'Defense Technologies' and then apply for weapon engineering. Some of the major institutes in India are:
1) Defense Institute of Advanced Technology, Pune,
2) Maharashtra Institute of Armament Technology, Pune
3) College of Military Engineering, Pune.
4) Defense Research and Development Organization. If you r wishing to go abroad (for which I am suggesting you as more career opportunities) are:
1) Cranfield Defense and Security, Shrivenham, UK
2) University of Leeds, UK
3) Taylors University, Malaysia
4) American Military University, USA
5) Naval Postgraduate School, USA
6) University of Wolverhampton, UK
7) Stevens Institute of Technology, USA
8) United States Naval Academy, USA
9) United States Military Academy, USA.
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Good luck buddy!

Ajay don't go with which stream is better. Go with what you want do after studying B.Tech. You may take following considerations. If you like computer, want to build crazy application or something related to software go with Computer Science engineering. If you like playing with machines/motors or electricity go ahead with Mechanical engineering. Or Electrical engineering. If you are India, after studying Electronics and communication engineering most of the students gets job/work in IT companies. Only few percent gets chance in electronics companies. So I would suggest, better to study Computer Science engineering. If you want to sail through vast sea go with Marine engineering. If you have not decided what you want to do after B.Tech, sit for one or two days and decided what you want to do. If still confused, select any of the engineering mentioned in point # 1, 2 and 3 (Computer Science, Electrical, Mechanical, or Electronics). Best of luck.