I was messing around in my gun room and decided to toss a magpul STR (milspec) stock on my MR556 to see how I liked it.  Went on fine but when it was locked down there was still a bit of movement.  Is this normal?  I thought that the RE was milspec sized on the HK?

The OEM HK MR556 receiver extension typically has two adjustable ball detents that can be extended or retracted to eliminate rotational play.  Have you tried to adjust them?

I saw those....I did not know that they could be adjusted.  I thought they were there for smoother operation.  How do they adjust?  I have an E1 stock on it and that locks tight of course when I screw it down.

They are there to tighten up (or loosen) the fit between the rail on the tube and the width of the cut for the rail in the stock.  Basically, the idea is to extend them, or retract them (which ever way the stock cutout requires) so there is enough pressure from the detents on the stock to enable it to slide successfully on the tube without any wobble between the two parts.  I forget the size of the hex wrench that you need to move them, but you can figure it out with a little bit of trial and error.

Thanks!  I will check that out.