Hello aspirant!
Hope you are doing well!

Yes, you can join DRDO after MSc physics. You need to have good rank in GATE / CSIR NET or SLET/SET exam it will be beneficial.

DRDO every year conducts a Scientist Entry Test (SET) on all India basis for the recruitment of scientists 'B' posts. A huge number of candidates appear for DRDO SET test.

.Candidates having first class degree in Science, Engineering Technology, Electronics, Communication, Mechanical Engineering and Computer Science can apply for the SET exam.

You need to be a citizen of India, and age limit is 28 years.

Then you have to qualify DRDO interview.