Hi shooters. I'm new at the cerebral aspect of shooting and hunting (more of a shooter so SD never really entered my mind). I see the definition and I think about one of the weapons I own, and I just wonder if I truly can apply the math and conclude that the setup I'm using is any better than a factory load when it comes to penetration. Leaving details out that are unimportant (so as to not seem self-serving more than anything.. I'm not here to say I have better stuff than anyone) when calculating SD, I do think that rather than using the diameter of the bullet you seated, you really need to understand that the bore diameter.. more specifically the diameter of the bullet as it LEAVES the barrel is the diameter that should be used to calculate SD when truly assessing/comparing theoretical performance. For example, the various flavours of Lee Enfield rifles that were chambered for the 303=British commonly vary quite a bit in their measured bore diameters. If you fire a 311 diam bullet through a tight barrel, besides the horrible things going on inside there, the bullet actually exits with a higher SD than it would if it were fired through a worn or loose barrel. So a question- if I buy 7.62 54r and pull the 180 gr 311 diam bullet out, neck-down the case and press in a 308 180gr bullet.. and fire it, does it exit with a SD that is any better than the factory 311 bullet? My question stems from the claim one person made that making proper rounds for a tight bore will result in "better ballistics" due to the fact that it is smaller diam. I would agree "better ballistics" due to the loss of unnecessary friction and thus higher MV, but I'm here to argue that the diameter ends up being exactly the same and since the weight is the same the SD's are equal. Am I off track here? or just thinking too much?