close but no cigar it has to do with B.C. of the diameter of the round. basically gravity pulls harder on the .308 then it does on a .277 bullet. because of the amount of surface space. not much but enough to make a difference. just about all .308 bullets in standard twist with factory ammo have a range zero of about 125 yards. the .270 typically has a range zero of about 175 yards. unless I am mistaken. I am very tired and have worked way too much this week. If i am wrong I am sure someone will come along and correct me.
Mostly right, but it doesn't make my statement wrong. The BC has more to do with weight/mass vs surface area, so the shape is more important to BC than the weight/mass, and gravity hasn't anything to do with it, really; check Galileo's experiment off the Leaning Tower of Pisa to show that weight/mass does not change how fast things fall, unless their shape starts catching air . The .270, being smaller diameter, is longer than the .308 for the same weight, making it essentially more streamlined. That slight advantage is less important at heavier bullet weights, though.
I agree, the .270 is theoretically "flatter" shooting. But the 30-06 isn't exactly shooting rainbow trajectories either, and can be zeroed where you want it to be, and shoot POI at the POA about as far out as most people should be allowed to shoot. The reality is, they shoot very close to the same trajectory at most hunting ranges.
Here is an example...close to the same weight, zeroed at 200 meters. Essentially the same trajectory and energy out to 500 meters with 160 grains.
.270 160 Gr. Nosler Partition +1.74 / 2646 / 2331 +0.00 / 2451 / 2000 -7.52 / 2266 / 1710 -21.79 / 2091 / 1456 -44.09 / 1926 / 1235
30-06 165 Gr. Nosler Partition +1.70 / 2667 / 2605 +0.00 / 2445 / 2190 -7.51 / 2237 / 1833 -21.97 / 2041 / 1526 -44.83 / 1858 / 1264
The lighter 130 grain gives the .270 a slight advantage, but the chart was using a Boat tail for the .270, and not for the 30-06.
.270 130 Gr. BTSP +1.38 / 2857 / 2356 +0.00 / 2625 / 1989 -6.46 / 2406 / 1670 -18.89 / 2199 / 1396 -38.52 / 2005 / 1161
30-06 130 Gr. SP +1.38 / 2860 / 2361 +0.00 / 2542 / 1865 -6.79 / 2248 / 1458 -20.49 / 1978 / 1129 -43.11 / 1733 / 866
So you see why I laugh a little when people start talking about how much flatter the .270 is; when they are essentially the same. I mean, the difference in drop is less than most people can group at a given range. The gun and shooter are FAR more important to how well the round shoots, than the caliber.