The 308 can be heavy and slow, with a rainbow-like trajectory and it could be said that its time has passed and it should gracefully retire like many NFL quarterbacks…
But, what if we as consumers could get the industry to put out more modern, and thus more capable, loads for the old workhorse cartridges? Let’s explore that concept…
Let’s look at two available bullets for these calibers so we can get a better picture for this story:
The well-known Hornady 6.5mm 140gr ELD-M has the following published BC values (The G7 Value is used for long range bullets as it tends to be more accurate in a ballistics calculator):
G1 - .646
G7 - .326
While the Berger 155gr Hybrid has the following published BC values:
G1 - .478
G7 - .245
Now we must define the problem that we wish to solve – in this case we wish to know which round is ballistically superior…
But what makes something "Ballistically superior"?
- Energy on Target?
- Lack of Wind Drift?
- Bullet Drop?
- How well the bullet performs in soft tissue?
- All of the above?
- And at what distance does this matter?
Defining our Intended Target
Shooting paper or ringing steel needs almost zero energy on target while punching through a cinderblock wall requires a good deal of energy and adequate bullet construction.
For this example, we will define our intended target as something that’s not necessarily hiding behind a wall but is a “soft target” such as a deer or human (allowing for hunting or self-defense/military applications). Knowing this, we will need a bullet to penetrate into the vitals of that organism and perform its intended task of soft tissue damage through hydrostatic shock and the dispensing of the bullet’s energy.
What determines how well a bullet performs?
- A bullet’s weight – Easy
- A bullet’s construction – Quality construction for game
- A bullet’s sectional density... But what the hell is that?
A Look at Sectional Density
In a nutshell, assuming the same construction and velocity at time of impact, a heavier bullet will push further into the soft, or hard, tissue than a lighter bullet. A tougher constructed bullet will penetrate further than something with a thinner jacket as well.
So that brings us to sectional density and why it matters, but here is a practical example of how much difference sectional density can actually influence penetration:
A gentleman named Arthur Langsford was an Australian gunsmith that brought "Squeeze Bore" technology into the limelight using 22LR rimfire ammunition. He called these rifles “extruders” and that’s exactly what they did. The idea was to shoot a 22LR into a 20, or even 17, caliber bore and it would force the bullet down to the diameter of the bore itself. The chamber used a special forcing cone and would actually extrude the bullet down to the smaller diameter....
“But why should we even care about Arthur Langsford or squeeze bore rimfire rifles? What does this have to do with the cost of tea in China right now Wes?”
It's because Arthur Langsford knew something a bunch of the world still doesn't - longer bullets and bullets that are “heavy for caliber” have a higher SECTIONAL DENSITY! Arthur's squeeze bore bullets would penetrate steel pipes that would normally just flatten a normal 22 LR bullet!
(if you want to read more about Mr. Langford)
So now that we know that sectional density matters for penetration and that a bullet that is “heavy for caliber” and long for caliber will penetrate deeper than a short stubby bullet at the same velocity let’s look at a 155gr 308 vs a 140gr 6.5 bullet lengths
Bullets lengths as measured and recorded by JBM Ballistics
We can see that the 6.5 bullet, while lighter, is longer and this should equate to a higher Sectional Density number as well. We find that this holds true as the Berger’s published sectional density is .233 while the Hornady comes in at .287. What this means is that the 30 caliber Berger bullet, given the same velocity and assumed toughness of bullet construction, will penetrate less than the 6.5mm Hornady bullet.
"But Wes, the 308 is going faster!"
You are correct! Which means that now we must define the other parameters for ballistic superiority!
Velocity and Energy
We’ve covered a bullet’s terminal performance and we still have some questions, like where does it matter? Are we shooting through the brush at 75-100 yards and hitting that soft tissue, or are we concerned about tissue damage @ 1200 yards? There is a big difference there!
This discussion here is centered on longer range engagement of soft tissue targets.
Let's arbitrarily set a distance range of 800-1200 yards for our theoretical engagement distance. So, we will need good terminal performance at this distance. Now we are concerned with energy and impact velocity at those distances.
If we take these two bullets and plug them into our, not so theoretical, cartridges: (A 20" 308 @2900 fps & my personal 20" 6.5CM @ 2657 fps) we can gather some ballistics data at distances of 800-1200 yards for comparison. Berger Bullets has an excellent ballistics calculator on their website that we will use for this purpose. Since we are mainly looking at extended distances we will use the G7 Ballistic Coefficient number for our calculations as it tends to be a more accurate drag model for longer ranges.
308 Winchester Data using the 155gr Berger Bullet in a 20” barrel @ 2900 FPS
6.5 Creedmoor Data using the 140gr Hornady Bullet in a 20” barrel @ 2657 FPS
Bullet manufacturers will generally state that they have designed the bullets to open up at a specified minimum velocity to ensure adequate expansion and terminal performance on soft bodied targets. We should keep in mind that both of these cartridge combinations are most likely well outside of the range that these types of bullets are designed for. While this velocity varies quite a bit from bullet to bullet, I'd say a good minimum velocity would be 1800 fps.
However, at 800 yards, BOTH of these cartridges and bullet/velocity combinations are below that speed threshold!
So, did we just show that BOTH of these cartridges are ballistically INFERIOR for this task?
Possibly, but let’s keep in mind our quarry. They tend to be soft and, generally speaking across the animal kingdom, easier to kill than many other species that walks this planet. So, let’s assume the bullet just has to hit with the same energy as a 357 Magnum running 158gr bullets at point blank range, something many would consider an ethical kill. This “energy minimum” is 550 ft/lbs of energy.
The Creedmoor is above 550 ft/lbs of energy out to 1100 yards while the 308 is above 550 ft/lbs of energy out to 1000 yards - both cartridges handily staying above this minimum at 800 yards for this example…
But we also have to hit our target!
Putting the Bullet in the “Boiler Room”
Now we must look at two very important factors - Wind Drift and Bullet Drop.
How do we deal with these things to ensure we actually hit our intended target?
Well, drop is the effect of the earth’s gravity on the projectile WHILE THE BULLET IS IN THE AIR (this parts important). A bullet which starts its life flying faster will be in the air less time and will go further before falling to the earth than a bullet of the same weight traveling slower, but there is another factor in that drop and that is the bullets Ballistic Coefficient (BC). A bullet with a higher BC, which starts its life traveling at the same speed, will drop less than a bullet with a similar weight but worse BC value over the same distance.
Example - a brick going at Mach 1 vs a spindly shaped bullet traveling at Mach 1 - both weighing the same - the brick will be more affected by wind resistance and will slow faster and fall to the ground in a shorter distance
So, we can deal with bullet drop in a couple of different ways:
- Shoot the bullet faster - More powder or longer barrel
- Shoot a higher BC bullet - A more aerodynamic bullet shape
Since our example is based around the 308 Winchester using the 155gr Berger and a 6.5 Creedmoor using the 140gr Hornady and both using 20" barrels, we can't increase the BC of the 308’s bullet and can't shoot the 6.5’s bullet any faster. This means only thing we can do about gravity is get our distance to target correct.
*Enter the laser range finder*
This tool, when used correctly, effectively negates any issue with bullet drop. The laser range finder (LRF) allows the shooter to gather a very accurate distance which can then be fed into their ballistic calculator or cross-referenced against their DOPE card (Data gathered On Previous Engagements).
Now the only thing we have to worry about for hitting our target is Wind Drift. This comes down to the shooter being able to read and call or “bracket” the wind along with how well the bullet slips through the wind (a function of the bullet’s B.C.).
Assuming that the shooter of either rifle can call the wind to within 1mph of its actual wind speed value then it is up to the bullet to get through the wind and to its intended target. This is where shooter error and a forgiving bullet can still make for an impact vs a miss.
Let’s look at how much each bullet will drift then:
@ 800 yards the 308 Winchester running the 155gr bullet will experience 1.87 MILs of Wind Drift in a 10 MPH cross wind (full value @ 9 o'clock)
@ 800 yards the 6.5 Creedmoor running the 140gr bullet will experience 1.48 MILS of Wind Drift in a 10 MPH cross wind (full value @ 9 o'clock)
The difference sounds minimal because we are talking in MILs which is scaled to the distance of 800 yards, but let's convert mils to inches...
1 mil is 3.6" @ 100 yards or 28.8" at 800 yards.
This means the total wind drift for each cartridge is:
1.87 Mils - 53.85" for the 155gr .308 bullet
1.48 Mils - 42.62” for the 140gr 6.5mm bullet
The Creedmoor will exhibit ~11" LESS wind drift at that distance which can allow you, the shooter, to be a little off on your wind call and potentially still hit your intended target.
"But Wes, what if you are off on your range finding doesn't that pose more of a problem for the 6.5 than the 308?"
That’s an excellent question, let's look into this more…
Say you lasered the bush 25 yards short of your intended target (775 vs 800). Would the faster 308 ,which only has 6.08 MILS of drop, still hit the target (assuming a 30" tall vital zone of a human torso) and would the 6.5 miss the target with its 6.46 mil drop?
The 308 will drop 5.75 Mils vs the 6.08 Mils you calculated - 160.43" vs 175.10" = 14.67" higher impact
The 6.5 will drop 6.14 Mils vs the 6.46 Mils you calculated - 171.31" vs 186.05" = 14.74" Higher impact
For this particular ranging error (which the distances were rectal plucked), both bullets would impact roughly the same height above the intended impact point - assuming you were aiming in the middle of the 30" impact zone they both would hit.
Now we can do a brief recap…
What makes something "Ballistically superior"?
- Energy on Target? - @ 800 yards they both have adequate energy
- Lack of Wind Drift? - The 6.5 has roughly 11" less wind drift @ 800 yards
- Bullet Drop? - The 308 has less total drop, but does it matter with a mild ranging error?
- How well the bullet performs in soft tissue? - ??? We have to make assumptions here…
- All of the above?
And at what distance does this matter? - is 1200 Yards MUCH DIFFERENT THAN 800 YARDS?
We need to make an assumption that the bullet will perform as intended in soft tissue when the energy is above 550 ft/lbs. If this is the case then the 6.5 will carry that energy another 100 yards past the 308 in this example.
But what happens to the other factors as these bullets slow down (remember the BC effects how quickly the bullet slows down as well!).
If we look at 1200 yards - past both of these cartridges’ energy minimum but a distance where both of these bullets would still get to, then we end up with the following:
Velocity - 308 is 1066 fps (marginal) 6.5 is 1250 fps (well above transonic speed)
Energy - 308 is 391 ft/lbs (159 ft/lbs less than our standard 550 ft/lbs), 6.5 is 486 ft/lbs (only 64 ft/lbs down)
Drop - 308 is 13.10 MILS, 6.5 is 12.69 MILS (Less drop than the 308 at this point)
Wind Drift - 308 is 3.4 MILS, 6.5 is 2.54 MILS (3.4-2.54 MILS = .86 Mils or 37.15" less drift)
AND the 6.5 will actually get to the target first with a flight time of 1.97 vs the 2.07 flight time for the 308.
"Well Wes I'm seeing what you’re saying and I'm sold on the 6.5 being superior!"
HANG ON THERE SQUIRT!
Most people can’t/don’t actually shoot LR due to range availability, inability to get good at reading the wind, etc., so let’s look at the numbers where a vast majority of guys are likely to be capable of shooting and hitting these theoretical soft targets - same 2 rifles, same 2 loads, but at 400 yards instead of 800-1200 yards.
At 400 yards:
Velocity - 308 is 2173fps, 6.5 is 2128 fps - 308 is faster here
Energy - 308 is 1626 ft/lbs, 6.5 is 1408 ft/lbs - 308 hits harder here
Drop - 308 is 1.88 MILS, 6.5 is 2.15 MILS - 308 drops less here
Wind Drift - 308 is .8 MILS, 6.5 is .66 MILS - The 6.5 drifts less here (but only 2.02" less)
Now let’s go back again to: WHAT IS BALLISTICALLY SUPERIOR?
At 400 yards a 308 Winchester shooting a fast 155gr Berger bullet would be superior in almost every way, however, at 800-1200 yards the short 6.5 Creedmoor shooting the 140gr Hornady would be superior in almost most every way.
Most shooters would be well served with a 308 Winchester running a fast 155gr Berger bullet for soft target work out to moderate distances. Both cartridges are excellent in their own right and I have, and enjoy, shooting rifles chambered in both.
The real answer for what is ballistically superior will come down to the end user’s goals and what they are asking these cartridges to accomplish.
- The 6.5 Creedmoor really shines with barrels of 24-25” but that can sometimes be more barrel length than some would want to use for a truck gun or with a suppressor.
- The 308 Winchester loses less velocity when you start going into the shorter barrel lengths and can perform very well with barrels as short as 16” which makes it ideal for use as a handy truck rifle or when using a suppressor.
There will always come another “better mousetrap” and cartridge designs are no different. To determine what works for you and your application I hope this article has provided a solid framework for how to determine what cartridge/rifle/bullet combination may serve you well in the future. Who knows, maybe something like a 20” barreled 7mm TCU rifle will do EXACTLY what you need it to do and scratch that custom itch as well!
REMEMBER:
- DEFINE THE PROBLEM YOU'RE TRYING TO SOLVE FIRST!
- Distance matters
- Sectional Density Matters on soft tissue
- BC, like diamonds, lasts forever while velocity, like beauty, is fleeting.
Outstanding read Wes!!!