For a round aperture, selecting a size defines both your focus and your brightness.  With Rectangular Apertures you can select your focus and your brightness separately.

Focus on the horizontal edges of your sight picture (top of the post and bottom of the target) is determined by how tall the aperture opening is.  Brightness is determined by the overall open area of the aperture (height x width).

Focus is improved as you decrease aperture height.  The limit on how small you can go is determined by when you start to see diffraction bands, which look like spirerwebs in the rectangular opening (or like dust balls in a round opening).  Generally, shooters seem to accept around 0.025" tall without being bothered by the spiderwebs, however the appearance of spiderwebs varies by shooter, depending on how close you hold your eye to the aperture, and how bright the light is.

The smallest size I make is 0.023" tall, however many people see spider webs at this point.

Once you have selected a height, you can control brightness by choosing the width of the aperture to give you the open area you want.  The limit on how wide you can go is driven by the need to keep the aperture opening smaller than the pupil of your eye.  If the aperture is wider than your pupil, your sight picture becomes sensitive to parallax error if you shift your head slightly off-center.  This is because you will have an image which is bounded on one side by the aperture, and on the other side by your pupil edge.

Generally, in a well lit environment where the sun is not shining directly in your eye, the human pupil is around 0.125" in diameter, however this varies by person.  The widest aperture I recommend should not exceed about 0.075" in width, leaving a margin of error for alignment with the pupil.

For these reasons, I make a series of apertures for the AR platform which are all 0.025" tall, and vary the width to achieve your desired brightness.  These include apertures which have the equivalent brightness of round apertures of 0.040", 0.042", 0.046", and 0.050" diameters.  They measure:

0.040" = 0.025" tall x 0.050" wide

0.042" = 0.025" tall x 0.055" wide

0.046" = 0.025" tall x 0.065" wide

0.050" = 0.025" tall x 0.075" wide

Additionally, I make a selection of apertures which vary the height slightly, to meet an individual shooter's needs.

For an M-1/M-1A, the aperture sizes are slightly larger, due to the greater eye relief, and due to the fact that the aperture inclines as you raise the sight, reducing the effective aperture cross section when shooting distances beyond 600 yards.  The aperture for the M-1 is 0.030" tall x 0.075" wide, giving the equivalent brightness of the 0.052" round aperture used in the NM hoods.  These apertures are drilled 0.002" off-center, preserving the 1/2 MOA adjustment by rotating the hood.  A smaller 0.042" equivalent size is also offered for improved focus, if you are only shooting out to 600 yards.

Apertures for AR-15 are offered in sizes equivalent in brightness to round apertures from 0.036”, to 0.052”. 

Apertures for M-1A/Garand NM sights are offered with an opening of 0.026" x 0.052" (equal brightness to a 0.042" round aperture) and 0.030" x 0.075" (equal brightness to a 0.052" round aperture).  Note, these are offered as hoods only, or as the entire rack/hood assembly.  Stoning the edges of the rack to fit your sight base will be required. 

ORDER