How does one distinguish between A1 and A2 style front sights?

Which is more common?

Will the same adjustment tools work on either? Why or why not?

A2 will be more common than A1.

A1 posts are round, A2 posts are square.

I think there's some difference between clicks per turn (A1 = 5, A2 = 4?) but I'm not sure.

Colt currently offers four different front sight posts for use on their AR-15/M16 family of weapons.  The front sight posts can be categorized according to their basic shape, (square or round) their profile, (straight or tapered) and their height, (short or tall.)  The height of the posts are measured from the top of the sight post down to the top of the “flange” of the sight post.

The sight post with the part number SP64507 has a short, square and straight configuration.  This post has a nominal height of 0.270”

SP64507



Part number SP64665 also has a short and square post, but differs from the above sight post in that it has a tapered shape to it; broader at the base and narrowing at the top.  The height of this post is also 0.270”.

SP64665



Shown below are the square, short/straight and short/tapered sight posts side by side for comparison.



The sight post associated with the M16/M16A1 series of rifles has a round shape and is also short and tapered.  The part number for this post is SP61706.  It is slightly shorter than the two posts described above with a height of 0.260”.

SP61706



The last sight post is Colt’s tall sight post which has a nominal height of 0.300”.  It has a square and straight configuration.  Its part number is SP62447.

SP62447



Here is a pic with the round sight post and the tall sight post side by side for comparison.




The original round sight post has five equally spaced notches around the sight post flange for elevation adjustment.  Each notch corresponds to 1 MOA of elevation (when used with a rifle length sight radius).  The other three sight posts all have four elevation notches on their flanges corresponding to 1.25 MOA of evlevation.




The two short and square sight posts have a longer threaded shank than the tall sight post and round sight post.






Bushmaster makes a front sight post that has a nominal height of 0.310”.

excellent