Good information about the rear sight hole diameters. But do check the the most current CMP games rule book as of 1-25-17 about "As Issued Military Rifles" That would be 4.2.1  (g)  The maximum aperture size requirement has been eliminated and the current rules now simply read "internal diameter of rear apertures may be modified."

Paul's drill information remains very valid, but you will not be disqualified for a larger rear aperture.

You can be DQ'd for lots of other funky stuff, but rear aperture size is not longer one of them.

We shot a CMP vintage match at 200 yards here in central WI today. Dark, overcast, cold, windy and light rain. I shot my 1898 with original unmodified 1901 sight. It works fine on a perfect bright clear calm summer day, but not in today's conditions. Sheesh! What a struggle. Managed a 92 and a 93 prone slow fire. That was with a high 7 and one low 8. The wire size drills will be un-boxed tomorrow!