New P-Rod AR stock loose

I have recently mounted an AR stock to my Marauder pistol using one of those adapter sleeves that fits around the round “cap” at the back of the gun through which you can make hammer adjustments. 

There’s a problem though. The threaded end “cap” into which the adapter screws seems to be loose. The result is an annoying amount of “play” in the stock, particularly when fully extended.

Is there a way to tighten that “cap”?

thanks. This is driving me crazy.
 
That made all the difference in the world, thanks. Do you have any idea whap happens if that screw is removed? I thought maybe if i could remove that cap and scuff it up a bit. I could increase the friction between the "walls" to hopefully keep things tight. It felt to me like that cap had the hammer spring behind it but i didn't know what else. 

Thanks again.