First you have broken Battle Armor figure. 
In this case, this is someone else's figure that had a question.  So I had to find an old spare one, but if your battle armor feature no longer works, this might work for you too.

3.) Pry it open with sharp object
(Don't cut yourself)

5.) Make sure the big part of the spring is pointing out.

7.) Put the spring and barrel assembly into the housing of the torso.  (Make sure the spring end is in the spot just like shown in the picture)

9.) Carefully put the chest onto the back with your finger holding the barrel through the window of the chest piece. (If you hear the spring jump, or it doesn't work right when you test it go back to step 3)

11.) Add some glue in the seams.  Wipe off any excess glue. 

13.) Put the arms back on.

2.) Turn in over and do the same on the other side.

4.) Put the spring in the open end.

6.) Lay the back of the figure down

8.) Twist the spring only one time around, stopping just past the double damage cuts on the barrel.

10.) Get some model glue to hold the sides together.

12.) Find a way to hold the body together, so it can dry overnight.