Accuracy Difference Between .308 and 30-06

Guys, I'm going to show my ignorance here, so be kind.

I have various .308s and various 30-06's. Obviously both shoot a .308 caliber bullet. Case capacity and case length are different of course, and there may be differences in twist rates on the barrels, but the projectile is still .308 for each. Since case capacity is different, the 30-06 can take more powder, but of course it doesn't have to be loaded all the way "full".

My confusion is that the .308 chambered rifle is considered to be slightly more accurate than the equivalent 30-06 chambered rifle. I'm confused because if the same bullet with same grain weight is loaded, to the same pressure, with the same amount of the same pressure for each, and shot through the same twist rate barrel, wouldn't we have exactly the same results in terms of velocity, energy, accuracy, etc.? (assume we can hold all other variables exactly equal: shooter, weather, etc., etc., etc.).

Or am I missing the boat completely here.

Awaiting your thoughts,

JGW

I do not have any first-hand experience with this but those I know and trust will say at longer ranges (1k yards) the .30-06 wins the accuracy game. You can add more powder to the load than .308 for a little extra speed.

Accuracy is consistency. Loads for most Garand-useful projectiles (150-180 grains) with mid-range powders come fairly close to filling a 308 case while they leave quite a bit of unused volume in a 30-06 case. That means the flame front in the propellant propagating out from the primer will burn more consistently in the 308 case than the 30-06. That translates to more consistent chamber pressures and burn rates between shots. Which leads to greater consistency in muzzle velocity, and therefore better accuracy downrange.

That lack of extra volume in the 308 case is the 308's shortcoming against the 30-06 for longer ranges, at least for handloaders (factories can play with customized propellants). 30-06 can use heavier projectiles, with better ballistic coefficients, than 308, b/c the extra projectile volume starts to eat into the available propellant space for projectiles >180 grains in the 308 case. 30-06 has no such problem with longer projectiles, , and has room to increase propellant volume even with smaller bullets.

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Originally Posted by Shug

Accuracy is consistency. Loads for most Garand-useful projectiles (150-180 grains) with mid-range powders come fairly close to filling a 308 case while they leave quite a bit of unused volume in a 30-06 case. That means the flame front in the propellant propagating out from the primer will burn more consistently in the 308 case than the 30-06. That translates to more consistent chamber pressures and burn rates between shots. Which leads to greater consistency in muzzle velocity, and therefore better accuracy downrange.

That lack of extra volume in the 308 case is the 308's shortcoming against the 30-06 for longer ranges, at least for handloaders (factories can play with customized propellants). 30-06 can use heavier projectiles, with better ballistic coefficients, than 308, b/c the extra projectile volume starts to eat into the available propellant space for projectiles >180 grains in the 308 case. 30-06 has no such problem with longer projectiles, , and has room to increase propellant volume even with smaller bullets.

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Thanks. This makes some sense to me. Would this possibly be the reason that match shooters in the late '50's - early 60's started moving away from 30-06 and toward .308? (that and maybe they were being trained on the M14 by then, not the M1)?

JGW

Is one more accurate than the other? IMO yes, but is it the round or the rifle? My 308 Garand is a joy to shoot and very accurate.
Since it has less recoil its easier to shoot consistently .

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Originally Posted by Orlando

Is one more accurate than the other? IMO yes, but is it the round or the rifle? My 308 Garand is a joy to shoot and very accurate.
Since it has less recoil its easier to shoot consistently .

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Orlando makes a good point. A 308 Garand with a poorly fitting stock, a loose trigger guard, or loose gas cylinder will probably be less accurate than a 30-06 with well-fit stock, trigger guard, and gas cylinder. And even if everything else is the same, the accuracy difference is probably within shooter margin of error for most recreational shooters.

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Originally Posted by Shug

Orlando test link makes a good point. A 308 Garand with a poorly fitting stock, a loose trigger guard, or loose gas cylinder will probably be less accurate than a 30-06 with well-fit stock, trigger guard, and gas cylinder. And even if everything else is the same, the accuracy difference is probably within shooter margin of error for most recreational shooters.

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This actually doesn't sound all that bad if I understand you correctly. So this is kind of a beginner-frendlier option between the two, right?

A lot of 30-06 shooters fill the air space in the '06 case with Kapok, or some thing similar to it. It keeps the powder from laying down, hence better and more consistent ignition.
Jon

I wonder if that has any effect on throat erosion? Most throat erosion is a result of gas cutting and the kapok probably isn't moving that fast through the throat. But it's forming a semi-permeable blockage that high pressure gas will be seeping around as it accelerates down the barrel.

If a beginner was asking me, I'd recommend .308. But mostly for the wider range of choices of commercial ammo and slightly lower cost than 30-06 these days now that milsurp 30-06 is a memory. Not many beginners or people with Garands are shooting 1000+yard matches where 30-06 really starts to shine over .308.

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Originally Posted by Shug

Accuracy is consistency. Loads for most Garand-useful projectiles (150-180 grains) with mid-range powders come fairly close to filling a 308 case while they leave quite a bit of unused volume in a 30-06 case.

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Why is there so much unused case capacity in a 30-06 round? It seems odd considering that the 300 win mag exists. Is it due to pressure issues?