Man, that's awfully long for a 151gr 308, and the boattail's length is way out there too. Core must end way short of the jacket's meplat. I'm looking at bullet dimensional drawings in Bryan Litz's 1st book, and seeing nothing with a boattail of that length. And even Berger's 190 VLD's OAL is shorter by several thou. So since Berger's 155.5 Fullbore has a G7 BC of .237, and their 185 LR BT's is .283, I'd guess your theoretical 151 should be just a little above that of the 155.5's. I'd also predict that a bullet of that length with a short core would be likely to become unstable a lot sooner than a shorter bullet whose jacket comes closer to being filled by the core.
