By running out of steam, I mean that the lighter bullet will not have as many foot-pounds of energy at long range. Initially, a lighter bullet may have more energy due to the higher velocity. Heavier bullets also tend to have a higher ballistic coefficient which helps downrange as well.
Using the JBM ballistics calculations, and Nosler's BC for a 150gr @ 3000fps and a 125gr bullet @3200fps (reasonable for a 30-06) and keeping all things equal, the 150 gr bullet still has more energy at the muzzle, but at 1000 yards, has over even more footlbs of energy, and its drop is about the LESS at that distance. The difference in these numbers will change depending on what bullet is used according to its BC. If you got a 125gr bullet that had a BC equivalent to a heavier bullet, then the lighter bullet would be better due to the higher BC + higher velocity. Heavier bullets also tend to buck the wind better as well. Looking at the chart that I made on the JBM site, the differences between the 125 gr BT and the 150 accubond are pretty good towards the 150gr, especially for wind drift and energy. Not sure why the 1000 yard drop said zero....I also don't know anyone that makes a 140gr 30cal bullet to use as an apples to apples comparison.
125gr Nosler BT
Range Velocity Energy Drop Windage Time
(yards) (ft/sec) (ft-lbs)(inches)(inches)(sec)
0 3200 2842 -1.5 0 0
100 2924.6 2373.9 33.6 0.8 0.098
200 2665.5 1972 64.5 3.2 0.206
300 2421.6 1627.6 90.6 7.5 0.324
400 2191.9 1333.4 110.7 13.9 0.454
500 1973.9 1081.4 123.6 22.8 0.598
600 1770.1 869.6 127.5 34.6 0.759
700 1582.1 694.7 120.2 49.6 0.938
800 1412.9 554 99 68.5 1.139
900 1267.5 445.9 60.4 91.5 1.364
1000 1149.6 366.8 0 118.8 1.613
For the 150 gr bullet
Range Velocity Energy Drop Windage Time
(yards) (ft/sec)(ft-lbs)(inches)(inches)(sec)
0 3000 2997.5 -1.5 0 0
100 2777.2 2568.8 33.2 0.7 0.104
200 2565.5 2192.1 63.3 2.9 0.216
300 2364.2 1861.6 88.2 6.7 0.338
400 2172.7 1572.2 106.8 12.4 0.471
500 1989.4 1318.1 118 20.2 0.615
600 1815.9 1098.3 120.4 30.4 0.773
700 1653.5 910.6 112.2 43.3 0.946
800 1503.2 752.6 91.3 59.2 1.137
900 1367.7 623 55 78.5 1.346
1000 1250.1 520.5 0 101.3 1.576