7.62NATO

You are correct it is a very small weak spring.... this again is going to sound dumb but the spring must be in the assembly in the correct direction or it will not return the pin.

Another thought you could/can pull the spring (carefully) to extend its length - but it still has to go in the assembly the correct direction. If I remember right it is a cone shaped spring??? When you can see the drawing/picture - you will see the correct orientation of the spring...

mike