Excessive windage
10JUL03
19AUG04
02SEP04

Often caused by the barrel being installed into the upper cockeyed. The barrel will turn as the barrel nut is tightened. Can also be caused by the FSB being mounted crooked on the barrel, the alignment pin, or its notch in the upper, being mis located, or a bent barrel. Sometimes, the front face of the upper, where the barrel seats, isn't square but that is rare. It is normal for the A2 rear sight to sit slightly rotated to the left but just slightly.

Excessive elevation
26JUL03
Hmm...Didn't know this about front sight bases
Think I have one of those mis-indexed barrels?(pics) Also has windage info.
2MAY04
detachable carry handles and FSB height differences
Height of flattop uppers and of corresponding parts and pieces
Busmaster .040" higher front sight post

If using Colt parts be aware that Colt uses a different height FSB on their barrels intended for use on flattops. These are the "F" marked barrels. There are also two different height Picatinny uppers floating around with corresponding detachable carry handles. Going to a taller front sight post is not a solution.

Damage to firing pin retaining pin
or
Bolt carrier "catching" or "hanging up".
22SEP03
16JUL03
02JAN04
06OCT03
26JAN04
30AUG04
M16 vs. AR15 firing pin differences?

When used with slotted bolt carriers, hooked hammers can catch the underside of the firing pin, especially if the firing pin has a larger spool on it like those intended for the M16. The hammer will force the pin rearward against the firing pin retaining pin during the battery stroke. With repetition, this will bend the firing pin retaining pin. Sometimes the firing pin retaining pin is a simple, non-hardened, cotter pin. Some of these bend rather quickly.

chrome plated bores
Colt barrel help please  

Chrome doesn't blue so you test shiny white spots near the muzzle and chamber to see if they change color when cold blue is applied.

to prevent