Help needed

[rods]

Hi new to this that being said i have a evolution tango down m4 i beleave well i took the stock off then the buffer tube then you have this round thing what ya can somtimes connect ya sling to anyway when i took this off a small spring flew of i beleave its to hold somthing together inside i think its called a d tension spring anyone know the correct name of it please or does anyone have 1 

 

[Sneaky]

I guessing you can buy any spring that can take place of the original spring.

 

[Lozart]

Hi new to this that being said i have a evolution tango down m4 i beleave well i took the stock off then the buffer tube then you have this round thing what ya can somtimes connect ya sling to anyway when i took this off a small spring flew of i beleave its to hold somthing together inside i think its called a d tension spring anyone know the correct name of it please or does anyone have 1 

Can you point to where the spring is missing?