Hello guys, I'm having a really strange problem with my new upper. It shoots 10 inches low at 50 yards, with the front sight post threaded all the the way in!  The upper is a standard Bushy, with an ARMS #40L SP rear sight and a YHM folding front sight.
  It groups nicely once you compensate for the elevation problem. I have compared the sights to another AR of mine and they appear to be the same height. Could I have over torqued the barrel and caused this problem?  Thanks, JT

 
 

Just a thought....
Is it appropriate type of front sight:
IE:
Rail mounted
or
GB mounted

The barrel has nothing to do w/it. You've apparently found a combo that doesn't work. Good job:O) Borrow another rear sight if possible.

Do you have a "F" marked A2 type fixed front sight?  Curious because I just ordered a front an upper with a "F" marked front sight.

...after re-reading your post, I changed my mind....

To clarify,

You have a YHM flip mount on the railed gasblock of a factory bushmaster upper?

- AG

Thanks for the responses, just to clarify, this is an upper that I put together. The front sight is the YHM gas block/folding front sight combo. I compared the height of this sight to a standard Bushy front sight and it appears to be about a 1/16" taller (measured from top of barrel to flat surface between ears where post threads into). I have a standard #40 BUIS that I will try next to see if there is a difference. JT