I've assembled over 60 AR-15's so far, but this is my first flattop with BUIS.
Here's my problem. I'm sighting in at 25 yards. I was quite low the first couple of rounds so I started screwing down the front sight post. The post is now bottomed out and I'm still 2-1/2" low!
My rear sight is a Midwest Industries. It seems to work fine, and there are no elevation adjustments possible.
The upper is a flattop made by Superior Arms in Iowa. The rail seems to be the same height as other flattops that I've compared it to.
The barrel is an M16A2. It was shortened to 16.25" and the original gasport was used to keep the rifle's longer sight radius. The gas port was opened slightly to allow operation in subzero weather.
The FSB is military issue.
I need to know if the M16A2 and the M4 carbine use different front sight bases? If so, could I have the wrong one on my rifle?