Update:
Went to the range today to see how my Remington 7400 30-06 autoloader would do with the Sims Barrel De-Resonator and (after loosening the forend stock's screw to just finger tight) achieved the following results:
August 5, 2005
12:10pm - No wind, but 94*F.
Target Distance: 100 meters
Ammo: Federal Premium Vital Shok 180-gr. Nosler Solid Base
Results: 4 shots within 5/8" from center to center.
Previously, with no Barrel De-Resonator and the stock's forend screw very tight:
June 2, 2005
10:30am - Slight East to West wind, and 81*F.
Target Distance: 100 meters
Ammo: Federal Premium Vital Shok 180-gr. Nosler Solid Base
Results: 4 shots within 1 3/4" from center to center
Seems that these two small adjustments cut the groups in half. The $19.95 Barrel De-Resonator was installed about 7/8" in front of the stock's forend screw, which seems to be the magic spot on this rifle.