IIRC, a year or year and a half ago, the NRA was re-writing/updating part of the Highpower Rules and there was an interim period where all bullet holes were supposed to be checked with a .30 cal plug. My understanding is that that temporary rule has since expired, once the updates were finished. According to the 2016 Highpower Rulebook:
14.3 How to Score—Hits will be scored based upon the diameter of the
bullet fired. When a shot hole’s leading edge comes in contact with the outside
of the “X” ring or other scoring rings of a target, it is given the higher value
(including the keyhole or tipped shots even though the hole is elongated to
the bullet’s length rather than being a circle of the bullet’s diameter). X’s
must be scored. A proper scoring gauge, overlay or magnifying glass may be
used to determine the value of close shots. The higher value will be allowed
in those cases where the flange on the gauge touches the scoring ring. Close
shots should be checked for the shooter’s benefit. Scoring gauges will comply
with the following dimensions:
Caliber Flange Diameter (+/- .001)
5.56mm .224
6mm .243
6.5mm .264
7mm .284
7.62mm .308
So the use of a .30 cal hole for all bullets on the e-targets seems to be a function of their increasing use in F-Class competitions. Whether this has been approved or even noticed by the NRA Rules Committee, I have no idea. I'd certainly imagine there was some involvement somewhere. Using a .30 cal hole value would without a doubt be an advantage (as compared to using a bullet diameter-specific plug) for someone shooting a .223 in F-TR, or a 6 mm or similar in F-Open. I have most definitely dropped points in matches with the .223 that would have been "10s" had a .30 cal plug been used to check them. However, I view the use of a "standard" .30 cal hole favorably not for that reason, but because what we should really be concerned about is where the center of the hole was located, not the diameter of the bullet the competitor was using. Why should a shot hole with the exact same center point on the target be a "9" for a competitor shooting a .223, and a "10" for someone using a .308? Standardization of the hole diameter value simply removes that as a variable for shooter using different bullet diameters, and effectively scores their shot based on the the center of the hole, as it should be.