As a long time .308WIN shooter, I was a very early adopter of 6.5CM (6.5 Creedmoor), smitten & seduced like so many, by the promise of all those sexy high-BC bullets and their wind-cheating magic. As such I had a ring-side seat to watch events unfold as 6.5CM rocketed in popularity and to observe the spectacle of the following cultural backlash against it.

I’ve enjoyed all the self deprecating memes over the years and listened to all the claims and opinions, and whilst it seems as though I’m a little late to the .308WIN vs 6.5CM debate, I have a lot of hard-earned first-hand experience of the things commonly being discussed, and the debate itself is showing no signs of calming down – so here’s my tuppence worth…

To try and be a fair as a possible and avoid either calibre being judged on the merits of a single bullet alone, I looked at Hornady’s entire published load data for each calibre and calculated the energy that each bullet/load generated, in joules.

It’s a useful exercise that reveals which projectile performs the best in each calibre. Is there indeed a sweet-spot for each calibre? This exercise will confirm it.

For .308WIN the highest performing load turned out to be the 208gr ELD-Match, which at 2,450fps generated 3,759 joules. For the 6.5CM it was the 135gr A-Tip Match, which at 3,000 fps generated 3,658 joules of energy. That's a 2% difference between the two highest performing loads - hardly chalk & cheese.

Had I simply picked two projectiles arbitrarily, there is a danger that I could have put a calibre at a false disadvantage.

It’s clear from this exercise that a number of different Hornady .308WIN projectiles emerge from the barrel with greater gusto than 6.5CM’s best projectile on it’s best day. Admittedly there’s not a lot in it, but technically .308 does have more energy at the muzzle than 6.5CM. So in my opinion that particular clubhouse ‘fact’ has been verified as true, if a little pedantic.

The second club house fact that you will often hear is that 6.5CM is ‘more efficient’. This breaks down into two parts: 1. Intelligent use of case capacity and how efficient the cartridge case geometry is at burning said propellant, and; 2. The BC of the projectile itself and how long it can hold on to its energy during the external ballistics phase.

In the 6.5CM, the 135gr A-Tip generates 3,658 joules of energy from a 46.0grn propellant charge. Which means that each grain of propellant generates 79.52 joules of energy.

In the .308WIN, the 208gr ELD-Match generates 3,759 joules of energy from a 44.6gr propellant charge. Which means that each grain of propellant generates 84.28 joules of energy.

This means that the 6.5CM burns its propellant with about 94% of the efficiency of the .308WIN. Again, not exactly an earth-shattering difference.

This confirms that .308WIN is doing more with each grain of propellant, and is therefore more efficient.

Of course, different propellants will produce different amounts of energy per grain. But remember we have selected each calibre's highest performing load/projectile, and subsequently the highest performing propellant for that projectile – so all that we can say is that .308WIN's highest performing load outperforms 6.5CM's highest performing load, in terms of propellant efficiency. So it's hard to see any evidence to support the claim that the 6.5CM is more efficient in this respect, at least.

So the argument that the 6.5CM case design is ‘more efficient’ in terms of powder burn clearly doesn’t seem to hold water. Ask those professing this greater efficiency what they mean exactly and you’ll most likely be met with vague mumblings, because there’s a very good chance they have no first-hand experience of the subject matter and they are simply parroting what they heard someone else saying.

There is then the assertion, that by seating the bullet out longer, the 6.5CMs case design is more efficient in that it makes better use of the available volume, as the tail of the bullet isn’t eating into the available case capacity. Whilst this is true, it does overlook the fact that the 6.5CM takes the .308WIN's shoulder and bumps it down, thereby reducing case capacity from 56gr to 52.5gr of H2O. In this respect the 6.5CM case is a bit like the opposite of an Ackley Improved cartridge – where a shoulder is sharpened in order to increase case capacity.

So whilst the 6.5CM does have the sharper shoulder which is so in vogue, and the tail of the bullet doesn't eat into the case capacity, pushing it down like this does result in a smaller capacity. You have to ask yourself what is the difference between a larger capacity and a bullet which eats into it – and a smaller capacity where the bullet doesn't eat into it? The bullet might not be eating into the 6.5CM's case capacity, but the sharper shoulder angle already has? Had the designers started with a .308WIN case and AI'd it's shoulder, thereby making it sharper and increasing the case capacity, then that case would have been a more efficient use of space. But that case would also result in a cartridge with a COAL greater than 2.800". In my opinion the .260REM AI makes more sense than the 6.5CM's design. Regardless, we can see from the real world results that the 6.5CM's case design yields no real world benefits - the .308WIN case design is simply burning its propellant with greater efficiency and the results can be measured at the muzzle.

But let’s take a look at the second part of the efficiency claim - that 6.5CM projectiles are more efficient in flight, and therefore retain more of their energy down range. The .308WIN’s 208gr ELD-Match projectile has a G1 BC of 0.690, whereas the 6.5CM’s 135gr A-Tip Match has a G1 BC of 0.637. It looks as though the .308WIN has the edge here, but again the difference is a hair's breadth.

On paper, judging by BCs alone, it looks as though the .308WINs projectile is going to be more efficient in flight. But lets plot both in Strelok Pro and see how the results pan out, all the way out to 1,000yds.

At the muzzle the .308WIN has 3,758 joules and the 6.5CM has 3,658 joules. At 1,000 yards the .308WIN has retained 1,200 joules and the 6.5CM 1,198 joules. So if you wanted to be super-pedantic you could argue that the 6.5CM is starting to close the gap on its disadvantage at 1,000 yards, from a 100 joule disadvantage at the muzzle to a 2 joule disadvantage. But such an argument really would be the epitome of splitting hairs. It’s also true to say that at no point between 0—1,000yds does the 6.5CM beat the .308WIN in terms of energy.

In fact, looking at the results, the two cartridges are so tight that they might as well be one-and-the-same, and I suspect that individual propellant lot numbers and environmental factors will result in far larger variations than the difference between the two calibres themselves.

So in my option, the often heard assertion that the 6.5CM is a more efficient design than the .308WIN has no basis in fact. The .308WIN is more efficient in terms of powder burn than the 6.5CM, and it’s optimum projectile has a higher BC which beats the 6.5CM’s optimum projectile at every distance between 0—1,000yds.

Then of course there is the fact that the .308WIN’s 208gr ELD-Match projectile is travelling at 2,450fps compared to the 6.5CM’s 135gr A-Tip travelling at 3,000fps. Everything else being equal, the .308WIN’s projectile will take longer to get to a set distance, and as such, gravity will act on it for longer resulting in a greater drop.

Most of the shooting that I do is either at known ranges or in situations where I have access to a LRF (laser range finder) device which helps me in the knowing. My scope has an elevation turret that allows me to dial-in corrections, and if my ballistics data is robust and proven, it really doesn’t make much odds to me whether I’m dialing-in a 6MILS correction or a 4MILS. There will of course be circumstances where it is impractical to know distance to target each time, and in those conditions a flatter shooting calibre will provide a greater margin of error.

But in my experience I am more likely to miss due to misreading the wind than I am due to getting my drop wrong. And a 208gr projectile that behaves like a freight train in the wind is preferable to a super-fast lighter weight projectile that gets blown around easily.

However, that super-fast projectile is in flight for a shorter period of time, and so the wind has a shorter opportunity to deflect it's path. So the question now becomes at what point does the shorter flight time of the super-lightweight projectile beat the obstinate, slow, freight train bullet?

That is the million dollar question, and something that ballistics aficionados spend a great deal of time considering.

So now we come to the factor that has probably sold more people on 6.5CM that any other – wind deflection. Whilst the .308WIN 208gr ELD-Match may have a higher BC, the combination of the 6.5CM's impressive BC and greater velocity promises that the 135gr A-Tip Match will take less wind.

The experienced shooter knows that there is no such thing as a dead certain wind-hold. Every wind-hold is a guestimate to a greater or lesser degree. In these circumstances the projectile which is the most forgiving, represents a competitive advantage. And it is this attribute of 6.5CM which is perhaps the most convincing. Indeed for a lot of competitive 6.5CM shooters this wind deflection chart is the only one that matters.