Al, using the JBM Ballistics site, the Sg numbers you note are what result from entering bullet geometry (entering the data) and hitting the calculate button . . . since McCoy used a "homogenous" bullet, these calculated Sg figures SHOULD be a little conservative (stability should be slightly better).
However, when using the typical .825" based bullets, it does appear that opting for a 1:13" twist for the 6mm would probably produce smaller aggs for the masses - this assumes the typical 66-68 Gr. bullet which is based upon the 0.825" long jacket. However, the best approach would be for the BR community to simply switch to bullets [of the same weights] based upon either the .790" or .750" long jackets - MV, thus RPM would remain unchanged, but relative stability for either would place the bullet soundly within the "magic window" - 1.4 to 1.9 Sg. This Sg range offers the best compromises betweeen displacement caused by CG offset and pitch / yaw. Your comment regarding Walt Bergers use of [likely] shorter, as well as lighter bullets IS interesting - another Hall of Famer, Fred Hauscuster (hope I got the spellin' correct), who used/invented the TALLDOG (a shortened 6BR), also used "short" 6mm bullets (I believe he used .750" jackets) and 1:14" twist barrels - these guys aren't exactly dumb & dumber! ;)
Having messed with this stuff for six full seasons now, it appears that keeping Sg within the 1.4 to 1.9 range is the way to go; BR shooters seem overly tempted by the exceptions! Mike Walker, in a small PS article (probably only two paragraphs), about 1987-88, stated (a paraphrase here) that he believed that the decision to settle on the 0.825" long jacket for the 68 Gr. 6mm bullets was a MISTAKE . . he recommenced 0.80" or shorter for the 14" twist. Our messing around with the numbers (we've used twist rates between 1:14 and 20) seems to offer imperical evidence to suppost this opinion . . . Sg in the 1.2 -1.3 area "work" for SOME barrels and in SOME conditions with SOME bullets , while the next bloke suffers continual frustration . . the SOME(ok,sum) of which is NOT all that good! ;) The most extreme example I can think of this past season was that Dave Tooley, who won the over-all TOP DOG honors at the NBRSA 1000 Yard Nationals, used my 187 GR. FB bullets, spun by a 1:14" twist barrel (Sg about 1.1 - at sea level & Std Conditions) - not what anyone would recommend . . and a few have tried to duplicate this model and failed!
Getting older every day has led me back to "better be safe than sorry" thinking - from now on, it's 1.4 - 1.9 Sg for ALL of my stuff !
Keep 'em ON the X! R.G.
