Here’s my question.

If you put an ARMS sleeve or ARMS 40 BUIS on a Bushmaster or RRA sometimes you have to get a taller front sight post, because the COLT front sight is higher(which is what ARMS designed their stuff for). Correct?

Okay, So is the reverse  true?  If you put a BMAS sight on a Colt or Colt spec gun you might need to get a shorter front sight post?

The reason I asked is because I just tried out a BMAS flip up rear sight I had, on my ZM LR-300 upper (which I’m assuming now is Colt spec). The sight is completely bottomed out and it still shoots low.  The front sight has a tritium insert so I’m not sure if it is designed to hold the glowing mass or the tip of the sight on your target.

I was planning on an ARMS 40 for this upper anyway. Do you think this sight will set higher than the BMAS unit?

From the makers I know that make to mil spec cause they sell to our military, you must have the correct ht. or the gov't will reject the weapon.  Colt (detach handle), KAC, and ARMS make to the correct ht. there may be others, but that's not certified to my knowledge.  If you go on a gov't spec weapon, the ht. dif.  problem doesn't exist. If you go onto an after market weapon, you might have to get an extended post,. but not always, it seems to be hit or miss, no pun intended.
Good Shootin, Jack