The following quote from a New Zealand web site sums the Whisper up nicely (
http://home.xtra.co.nz/hosts/suppress/whisper.htm):
The payload for the subsonic .300 Whisper ammunition is preferably in the 240-250 grain weight. Using anything under 220 grains is a waste of time as both energy & "yawing" effect diminish quickly with lighter projectiles. Considering the approximately 1040fps bullet speed needed for subsonic load only the projectile's weight can create sufficient energy. The "yawing" refers to the tumbling of these long 240 & 250 grain projectiles immediately after impact - a major part of the Whisper concept, disreguarded or misunderstood by many. While they are stable in flight, even a cardboard target clearly shows the tendency of these bullets to tumble on impact, thus creating wounds far in excess of their "paper" ballistics. It is futile to look for the magic bullet that expands or mushrooms at subsonic speed - such a thing in a .30 cal sectional density doesn't exist, nor will experiments with cast lead projectiles lead to greatly improved result.
However, a hard & solid 240 grain Sierra Matchking turning over lengthwise is equivalent to a seven times calibre expansion! These high ballistic coefficient projectiles retain energy as well as sub MOE accuracy out to 200 yards & more. While the bullet leaves the muzzle at 1040 fps, at 100 yards it loses only 3%, travelling at 1011fps & at 200 yards it still keeps on motoring with a solid 984fps. As velocity is retained, so is the energy; muzzle 576 ft/ibls, 100 yards 544 ft/lbs & 200 yards 517 ft/lbs. Only beyond 250 yards does the energy start to drop below the 500 ft/lbs mark.
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