Gus Norcross, originally trained on National Match rifles and pistols by the National Guard Marksmanship Training Unit specializes in Garands, M14s and 1911s at his small shop on the coast of Maine. His website is www.angusarms.com. He will be offering gunsmithing tips and tricks on Fridays.
Here is a typical scenario: You get a killer deal on a used AR-style carbine at the local gun emporium and you rush to the range to test-fire and zero your new prize. Upon dialing in a typical 25-yard military zero with the detachable carry handle or a folding backup rear sight you notice the front sight post is screwed almost completely out of the front sight housing.
The problem here is that the manufacturer assembled the rifle with a standard early front sight assembly designed for rifles with integral carry handles rather than one meant for flat top receivers. This situation may not be a real issue as long as the rifle zeroes but it certainly looks funny and it isn't right.
When Colt designed the original flat top upper receivers for the M4 carbine they couldn't just hack off the carry handle and install a rail. The top of the receiver had to be strengthened to maintain structural integrity.
This means the detachable carry handle sat a bit higher than a normal integral carry handle and the front sight was a bit too low. As Patrick Sweeney points out in his book Gunsmithing The AR-15, Colt could have modified the carry handle for an easy fix, but instead they chose to alter the height of the front sight housing.
These new front sights were stamped with a letter "F" to differentiate them from the older models. Front sight deck height will undoubtedly vary slightly between commercial manufacturers but the difference between old and new runs in the .080"-.090" range.
Turning the front sight counterclockwise to raise it may cause the sight flange to rise above flush with the flat surface of the housing.
At this point, the sight post may only be retained by a few threads. Replacing a pinned front sight housing is possible but difficult without proper tooling. The easy fix here is to purchase a higher front sight post. Bushmaster Firearms sells one that is .040" higher than a standard A2 style, part number F1003228, available from their website for $5.50.
Next time a rifle manufacturer claims its M4 carbine clone is "mil-spec" and they don't use the higher front sight housing the military issues, you'll know there is a bit of BS involved in their advertising.
Front sights with \"F\"
Front sights with the proper deck height to match M4 upper receivers are marked with the letter "F". Earlier sights were made for the fixed-handle A1 or A2 versions
You may find the front sight must be almost completely backed out when the front sight tower and upper receiver are mismatched.