LOL..NoMercy, I have a feeling we have us a reader with no practical experience to back up what he thinks is reality.
1: zrex, could you give the class your reasoning on how a cartridge case in ANY way effects SD (Sectional Density) or for that matter how SD (Sectional Density) effects bullet flight in any way whatsoever?
2: One of the reasons medium and short action cartridges are more accurate has absolutely NOTHING to do with the cartridge itself but in fact the ACTION itself! Shorter action means STIFFER action. Less "flex" means more accuracy.
3: The pressure curves in medium and short action cartridges are different and reach maximum pressure quicker, throttling up bullet speeds a bit faster. We are talking dang near nano seconds here. Your efficiency argument is pretty much moot since, as RR already stated, burn rate is the director here not cartridge shape. You put a faster powder in a smaller space then yes you will have faster bullet speeds because there is unfilled space in the longer cartridge that has to pressurize before bullet movement. Again, nano seconds but yes there is a difference.
4: The 6mm bullet is by far one of the best long range PAPER PUNCHERS out there no question about it. But there is a reason it's not used for long range big game hunting or for Long Range Military uses. Retained energy just sin't there.
zrex, you are sitting here arguing with 3 or 4 folks that have PRACTICAL knowledge in the long range shooting world. Not just reading about it. Nomercy is a top shelf engineer, RR has been in the long range game for over 25 years or so. Jeep Kid works for a company that DESIGNS long range custom firearms! And I myself have been in the shooting/hunting game longer than all 3 of them combined! Over the years I have been asked to test ammo from dang near EVERY American ammo manufacturer. I've been wildcatting cartridges since the 60's. So yeah we have just a tiny bit of a REAL clue there kiddo.