The .308 gets much of its oft vaunted “efficiency” through the use of higher MAP than the .30-06 (62KPSI vs 60KPSI).
There's really four ways you can increase efficiency:
Smaller case
Higher pressure
Shoulder that catches more powder
Longer barrel
Relative to the .30-06, the .308 improves on all four (by being short action, you get more usable barrel for the same nominal length/weight). It's not any one that's the source of the benefit.
If velocity is the goal, “efficiency” is at least secondary and maybe further down the line.
Generally speaking, a "shoulder that catches more powder” means a “larger case capacity” that is counter-productive to “efficiency”, whether you measure it in terms of fps/grain or fpe/grain.
A “longer barrel” has nothing to do with cartridge efficiency. Increasing or decreasing barrel length may change velocity but it does nothing to change a cartridge’s innate characteristics.
“Higher pressure” means more powder per volume with diminishing velocity per grain of powder, which REDUCES ”efficiency”. Think otherwise? Compare the pressure and “efficiency” of standard loads with subsonics. The 300 Blackout hits a fps/grain “efficiency” of 209 with 110g bullet and a pressure of 18,600 PSI (Hodgdon data). The .308 Win is WAY oversized by comparison, requiring a larger, heavier rifle and failing to achieve the same “efficiency”. That said, using “higher pressure” in cartridge “A” and lower pressure in cartridge “B” can skew “efficiency” measurements in favor of cartridge “A” – the .308 Win in this case. Load the .308 Win and .30-06 to the same pressures and the .30-06 wins the velocity race.
“Smaller cases” do tend to increase cartridge “efficiency” (fps/grain or fpe/grain). A .300 Blackout can push a 110g bullet to 2474fps with 21.0g powder, according to Hodgdon, giving it a fps/grain “efficiency of 117.8. By contrast, and again using Hodgdon data, a .308 is stuck in the high 70’s with the same bullet weight. Switching to a 150g bullet, the .300 Blackout comes in at a high of about 120 while the .308 rates in the 50’s and 60’s. If “efficiency” is your goal, why don’t you shoot a .300 Blackout or something similar? (Hint: Because “efficiency" is NOT your primary goal.)
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“Efficiency” is only important to the degree that “efficiency” is the goal.
Call me crazy, but it should be a BIG part of the goal. Otherwise we'd all be shooting .30-378 Weatherbys or .30-338 Lapuas and making fun of jwall's puny .30-06 and it's pathetic external ballistics. All you're going to see is the dog's rear, jwall! See how stupid that is?
A sensible hunter has an animal (or animals) they want to kill, knows what bullets will provide good terminal performance on that animal and at what velocities, knows what trajectory they need, and then is looking for an efficient cartridge that gets that done with the minimum rifle weight and recoil. That analysis almost always favors the factors I listed above, as well as smaller calibers with high sectional density premium bullets. Looking at those factors .30-06 pretty much is never the right answer. It is however what great grandpa carried in the trenches in France. But grandpapa hated the darn thing (an inaccurate metal butt plate .30-06 with an eternity-long lock time - horrible idea) and I don't care for it one way or the other.
For me, “efficiency” in hunting rounds is a minor part of the “goal”. My .300 Blackout is “efficient”. So are ALL my other .308 caliber rifles, be they .30-30, .308 Win, .30-06 or .300 WM. By “efficient”, I mean they burn a reasonable amount of powder for the velocity they produce. Whether I spend $0.14 to fill a .308 case or $0.18 to fill a .30-06, the difference in powder cost is minor compared to the differences in bullet costs. A cheap FMJ plinking bullet might cost $0.21 each while my preferred hunting bullets cost several times that. A 168 grain Barnes TTSX, for example, currently runs $0.76 each at Midwayusa. When the difference between $0.95 and $0.99 handloads breaks the bank (Barnes 168g TTSX in .308 Win and .30-06 respectively), it is long past time to give up shooting altogether.
Your grandpa’s battle rifle has nothing to do with the relative merits of the .308 Win and .30-06. Maybe you should start a thread on the merits of such rifles compared to modern commercial rifles.
Coyote Hunter - NRA Patriot Life, NRA Whittington Center Life, GOA, DAD - and I VOTE!
No, I'm not a Ruger bigot - just an unabashed fan of their revolvers, M77's and #1's.
