Some recent discussion on the Florida Smallbore forum got me to thinking seriously about this topic.

Result . . . I did some refining of the data originally published by Lester Karas in an old issue of  'Precision Shooting', plus a little practical testing using my own target rifles . . . an Anschutz 1907 with a 10" Medusa bloop tube, and an H&R M.12.    I also re-calculated and extended the Karas data to cover the 'Eye-to-Front Sight' distances more common for current smallbore practice using 'bloop tubes' or Hammer-Shark radius extenders, and also to cover the range of stock aperture sizes most readily available from our regular suppliers.

Similarly I reviewed the NRA Smallbore Handbook target size section to obtain aiming bull data.

Using this data, I ran several tests on available targets on my 50 yard home range to verify its' validity . . . it appears to be quite valid for this old fellow, with fading eyesight, tri-focals and an Anschutz 1.5 magnification lens in his rearsight.    I also did some tests without the rear lens but with both 0.25 and 0.50 diopter front lenses in place . . . again satisfactory !

To Determine Correct Front Aperture Size - (Inches or Metric)

1.      Obtain 'Optimum Aperture' size in MOA from Table 1 for the Target / Distance in use

2.      Using the 'Eye-to-Front Sight' distance column in Table 2, applicable to you and your rifle, find the
        'next-smaller' aperture diameter corresponding to this 'Optimum MOA'.    This aperture diameter
        will give the maximum possible clarity of view of front aperture and aiming bull at that range.

3.      If 'less white' around the aiming bull is your preference, then the aperture size can be reduced to no
        smaller than the MOA value shown in the 'Absolute Min.' column of Table 1.    This is a practical limit
        at which fuzziness of the 'white junction' between inner aperture edge and the aiming bull becomes a major
        problem.

4.      This approach works for both "pure' Iron sights, or for those with a corrective lens in the rear sight.    If a
        magnifying lens is used in the front sight, (typically either 0.25 or 0.50 diopter), then the 'True Diameter'
        in MOA of the aiming bull should be multiplied by 1.25 or 1.50 respectively.    The resultant value should
        then be increased by a value of 6.0 or 5.0 to obtain the 'Optimum' or 'Absolute Minimum' values for the
        aperture.

5.      You will note that Table 2 is colour coded in bands
                - the 'Blue' areas are outside the range of normal (?) sighting
                - the 'Black' area applies primarily to Conventional and Metric NRA / International targets
                - the 'Red' area would normally apply to Long-Range / Mini-Palma shooting
               - the 'Green' area would normally apply to 'Sporter' air rifles at 10 metres
              - the 'Yellow' area would normally apply to Rimfire Sporter rifles in CMP events

TABLE 1

TABLE 2