Hi, 

Firstly, 516 is a good score and you may get a college. There is a chance but still we can't say anything with 100% authority because we don't know what will the cut off of different colleges. If your score would have been above 550, then we could've that you will definitely  get a college but still don't lose heart. It's a nice score and you have worked hard. Hope for the best. 

Secondly, there is no defence quota in Neet. You have to score high marks and get good rank in order to get a good government  college. 

Good luck!