BC is pretty much proportional to Sectional Density, which in a given caliber means proportional to weight.... Therefore going from a weight of 15 gr. to 18 gr. should increase the BC from 0.075 (assuming that is correct) to about 0.090.... Our current theory is that Meplats of 50% of caliber and less have virtually the same BC, but as the Meplat diameter increases above 50%, the drag does also, and so the BC drops.... Corbin dies typically have about a 50% Meplat, probably not by accident.... .... Going much smaller makes the ejection pin too thin and prone to breakage in any case, particularly if it is a HP pin.... Slugs of typical Corbin shape (50% Meplat, 2R tangent Ogive, parallel body) should be good up to about Mach 0.9 (about 1000 fps) before the drag starts to increase quickly.... Above that velocity, you can expect wind drift to increase, not decrease as you might expect....Nick's website says that his 15 gr. is OK in an LW barrel, which is 17.7" twist.... Adding 3 gr. to the weight (in the parallel body) will add about 0.042" to the length, and may require a faster twist.... The 17.7" twist of the LW gives a stability of 1.4 for Nick's 0.270" long 15 gr. (from the Kolbe Twist Calculator.... 1.5 is recommended, 1.0 is unstable)... Increasing the length to 0.312" requires a 15.5" twist for the same stability.... You might get away with a 16" twist, slower than that could be asking for trouble with an 18 gr. slug....Required pressure times barrel length to reach the same velocity would increase about 20% going from a 15 gr. slug to an 18 gr.... assuming the same port size and valve dwell.... ie you can increase one or both to get there.... or valve dwell if you are not running wide open now (at a loss in efficiency)....Bob
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