Usually when comparing cartridges, the one with a larger case volume and smaller bullet wins. This is kind of apples to oranges. Both of those cartridges would have to be built on the AR10 platform. I dont think a 6.8 can outperform anything in that size.

 

For people that don't have a lot of knowledge and understanding about ballistics, I've often used the .243 as a very rudimentary comparison of the 6.8 performance out to 100 yards, which is most people's normal shooting distance. I explain that the 6.8 delivers similar energy with a similar bullet weight (frequently slightly heavier) than the .243 for the first 100 yards and does so with less recoil and muzzle blast due to it's smaller cartridge and then highlight it's benefits for young/small shooters.

Since the .243 is generally revered, I often receive nods of approval and continued interest. I leave out the barrel length details as that usually begins to cloud the discussion.

 

Very interesting, thanks for posting. If the 20”-16” barrel numbers were listed as from the 6.8 no one would think twice. A lot less powder and recoil for a better result.

Is the 6.8 SPC chipping away at the pedestal of the mighty .243?

 

All I can say is I own a BHW 24" gun. Running the 90 gr gold dots I can keep up with the 243 100 gr. Not bad for a puny little 6.8

 

To Fudds I just say: it’s a mini 270 to fit in an AR…

I always get some kind of response like: 270 is great deer round!
From which, depending on the level of Fudd and my desire or lack thereof to engage more, I will mostly just smile and nod.

 

What do you consider a Fudd?

 

I'd be more interested in the .243 vs the 6.8x51?

 

I don't really think of them as having much overlap. I have always considered the 243 a good varmint and predator cartridge that is just "OK" for deer hunting. I know that some people love it, but when I was young my tiny brain imprinted on the 270 and up as "real hunting cartridges". Heck, it has taken years for me to accept 6.5mm!

On the other hand, I consider the 6.8x51 to be essentially two different cartridges that fit in the same rifle: the High-Pressure and Normal-Pressure variants. The high-pressure variant will be especially useful for those who hunt suppressed and want a short barrel. SIG has posted velocity specs for its ammo on its website. It will pretty much match a traditional .270 in a shorter package (16"barrel). The ballistics will match the 270 WSM with a 24 inch barrel, but likely cost more per shot using factory ammo. The 6.8x51 has a throat design with a (short) section of moderately tight freebore, so it may be less picky than the 270 and 270WSM when it comes to ammo it likes.

277 SIG FURY, 150GR HYBRID HUNTER

Featuring NOSLER Accubond Bonded Core Bullets.

www.sigsauer.com

By loading closer to 65,000psi in an all-brass case, 6.8x5 will slightly exceed the performance of the 7mm-08, one of my favorite cartridges. The "slightly exceed" part is because even at 65,000psi, the 6.8x51 will be running at higher pressure and hold slightly more powder. The 6.8x51 has less body taper, a longer body, and steeper shoulder than the normal 7mm-08. It is really closer to the Ackley Improved 7mm-08.

277 SIG FURY, 135GR FMJ , ELITE BALL

Elite Match Grade FMG Ammunition

www.sigsauer.com

 

Curiosity got the better of me as the .243 is highly thought of as a multipurpose cartridge but it has always had a much better velocity rating than the 6.8 but I was more interested in the results with comparable barrel lengths.
Interesting results below:
243 Winchester- Effect of barrel length on velocity

It appears the 6mm Creedmore does better in a shortened barrel.
6 Creedmoor barrel length versus muzzle velocity (31 to 17 inches)

Your missing one key element. Terminal performance. Most people get far too hung up on exterior ballistics (velocity, kinetic energy, drop, drift etc.) while assuming terminal performance is similar or the same between projectile diameters and form factors. It is NOT. Just as exterior ballistics favor 6mm and 6.5mm long form factor projectiles, terminal performance favors shorter 6.8 and 7mm projectiles.

There is NO FREE LUNCH. Projectiles that are long, slender that fly through the air well also slip through the target body quite well (aka they do notably less damage). The opposite is true for larger diameter projectiles. The trade off is exterior ballistic performance vs. terminal performance. 6.8mm was found to have the best compromise for combat purposes in killing 150 to 250 lb humans (and consequently works well on animals that size range).

Take a look at the measured energy transfer of these four projectiles all fired from the same distance into the same ordnance grade gelatin from Brass Fetcher:

Notice the area under the curve between 6.5 Grendel and 5.56mm? Not much difference! The 6.5 Grendel cannot easily transfer it's energy to the gelatin (these are live shots with electronic load cells under the gel blocks measuring the pressure wave in real time imparted to the gelatin). If the pressure wave is sufficient, it will cause enlargement of the permanent wound cavity made by the bullet as it cuts or crushes tissue. The pressure wave follows immediately behind the projectile.

The bigger the caliber, the better the transfer. Just look at .50 Beowulf! It's a beast. Now, obviously .50 Beowulf isn't very aerodynamic and has a relatively low velocity, so it's not a cartridge that is suited to anything but close range. That's the trade off, it's not a good flyer, it is a really good hitter up close.

Here's the result of barrier blind through auto glass. Note that all three (5.56, 6.5, 6.8) are TSX bullets, about as apples to apples as you can get. 6.5 Grendel did a decent job as did 6.8 SPC, .50 Beowulf is no doubt the top performer. But 6.8 still produces notably higher peak energy transfer than 6.5mm or 5.56mm, which means there's more peak stress on soft tissue that may cause additional tearing of already damaged tissue by the pressure wave following the bullet at is creates the permanent wound cavity.

For reference, shorter projectiles are inherently more stable than longer more slender projectiles, this is one of the reasons why 6.8 SPC bullets perform more consistently compared to 6.5mm bullets when passing through barriers, because the form factors are different (meaning 6.5mm destabilizes more easily, you get more variation shot to shot on impact). 6.5mm has a tendency to completely destabilize and tumble when passing through the barrier, dumping all of it's energy too early and failing to penetrate adequately. I would make the claim that it will be inherently less consistent shot to shot on game animals, especially those with heavy bone structures or when firing through brush.

Also note that the slope of the nose section affects drag when penetrating the target, not just when flying through the air. 6.8mm projectiles in the shorter form factor tend to have shorter noses and a larger diameter, implying the slope is steeper than typical 6.5mm or 6mm low drag bullets. Low drag in air = low drag in target. High drag in air = higher drag in target = more energy transfer. Again, it's a balancing act between loss of velocity, drop and drift vs. optimal drag upon impact.

If you want a good hitter, 6.8mm and 7mm performed the best at the velocities we can achieve out of barrels ranging from 12-18". If you want a good flyer, 6.5mm and 6mm provide the best exterior ballistics but lack terminal performance and barrier blind performance comparatively, not just because of the smaller diameter, but because of the overall bullet shape.

So the real question, what's your intended use? Target shooting or killing things? If you want to kill things (e.g. hunting or duty rifle), 6.8 is still the better choice even it's not 6.8 SPC, you can consider the new .277 Sig Fury if you want .270 Winchester performance out of a 16" barrel. There's also the beast of a cartridge 6.8 Western for big game / long range. If you want target shooting, well look to the 6 and 6.5mm cartridges as they are optimal flyers for target shooting and paper punching.

Any of these projectiles are lethal, but it's a matter of how consistently and how fast they achieve lethality. Bigger is better in that case.

 

Your missing one key element. Terminal performance. Most people get far too hung up on exterior ballistics (velocity, kinetic energy, drop, drift etc.) while assuming terminal performance is similar or the same between projectile diameters and form factors. It is NOT. Just as exterior ballistics favor 6mm and 6.5mm long form factor projectiles, terminal performance favors shorter 6.8 and 7mm projectiles.

There is NO FREE LUNCH. Projectiles that are long, slender that fly through the air well also slip through the target body quite well (aka they do notably less damage). The opposite is true for larger diameter projectiles. The trade off is exterior ballistic performance vs. terminal performance. 6.8mm was found to have the best compromise for combat purposes in killing 150 to 250 lb humans (and consequently works well on animals that size range).

Take a look at the measured energy transfer of these four projectiles all fired from the same distance into the same ordnance grade gelatin from Brass Fetcher:

View attachment 76713

Notice the area under the curve between 6.5 Grendel and 5.56mm? Not much difference! The 6.5 Grendel cannot easily transfer it's energy to the gelatin (these are live shots with electronic load cells under the gel blocks measuring the pressure wave in real time imparted to the gelatin). If the pressure wave is sufficient, it will cause enlargement of the permanent wound cavity made by the bullet as it cuts or crushes tissue. The pressure wave follows immediately behind the projectile.

The bigger the caliber, the better the transfer. Just look at .50 Beowulf! It's a beast. Now, obviously .50 Beowulf isn't very aerodynamic and has a relatively low velocity, so it's not a cartridge that is suited to anything but close range. That's the trade off, it's not a good flyer, it is a really good hitter up close.

View attachment 76714

Here's the result of barrier blind through auto glass. Note that all three (5.56, 6.5, 6.8) are TSX bullets, about as apples to apples as you can get. 6.5 Grendel did a decent job as did 6.8 SPC, .50 Beowulf is no doubt the top performer. But 6.8 still produces notably higher peak energy transfer than 6.5mm or 5.56mm, which means there's more peak stress on soft tissue that may cause additional tearing of already damaged tissue by the pressure wave following the bullet at is creates the permanent wound cavity.

For reference, shorter projectiles are inherently more stable than longer more slender projectiles, this is one of the reasons why 6.8 SPC bullets perform more consistently compared to 6.5mm bullets when passing through barriers, because the form factors are different (meaning 6.5mm destabilizes more easily, you get more variation shot to shot on impact). 6.5mm has a tendency to completely destabilize and tumble when passing through the barrier, dumping all of it's energy too early and failing to penetrate adequately. I would make the claim that it will be inherently less consistent shot to shot on game animals, especially those with heavy bone structures or when firing through brush.

Also note that the slope of the nose section affects drag when penetrating the target, not just when flying through the air. 6.8mm projectiles in the shorter form factor tend to have shorter noses and a larger diameter, implying the slope is steeper than typical 6.5mm or 6mm low drag bullets. Low drag in air = low drag in target. High drag in air = higher drag in target = more energy transfer. Again, it's a balancing act between loss of velocity, drop and drift vs. optimal drag upon impact.

View attachment 76715

If you want a good hitter, 6.8mm and 7mm performed the best at the velocities we can achieve out of barrels ranging from 12-18". If you want a good flyer, 6.5mm and 6mm provide the best exterior ballistics but lack terminal performance and barrier blind performance comparatively, not just because of the smaller diameter, but because of the overall bullet shape.

So the real question, what's your intended use? Target shooting or killing things? If you want to kill things (e.g. hunting or duty rifle), 6.8 is still the better choice even it's not 6.8 SPC, you can consider the new .277 Sig Fury if you want .270 Winchester performance out of a 16" barrel. There's also the beast of a cartridge 6.8 Western for big game / long range. If you want target shooting, well look to the 6 and 6.5mm cartridges as they are optimal flyers for target shooting and paper punching.

Any of these projectiles are lethal, but it's a matter of how consistently and how fast they achieve lethality. Bigger is better in that case.

Its amazing how little some people know about ballistics. I posted a video of me shooting ballistics gel with a 143 eldx out of 6.5 creedmoor and a 178eldx from my 308. 6.5 was factory ammo and the 308 were handloads, they were going about the same velocity. So 178 grain 30 cal going the same speed as a 143 6.5. Obviously the 308 did more damage. I showed the the gel block cut in half and the 308 wound channel was larger. Some 6.5 fanatic comments that there is now way the 308 did more damage. I tried to explain the ballistics to him but he just kept commenting paragraphs of non sense about how the 6.5 is better in every scenerio. I never said the 308 was better than 6.5 overall, I said for the average hunter shooting inside 300 yards that the 308 would perform better but this guy just couldnt grasp simple terminal ballistics. I just stopped replying, arguing with stupid people is a waste of breath.

 

And that's why 6.5 Grendel fans are fanatical fans of 6.5 Grendel. It has it's merits, it is lethal, so if 5.56. That doesn't make it IDEAL for specific applications or some physics defying bullet. When you need fire power, aka the ability to destroy targets, especially those behind common types of barriers, the larger calibers in a shorter form factor really shine and perform well (6.8 and 7mm) and at realistic ranges people can actually shoot at under far from ideal field conditions.

6mm and 6.5mm projectiles absolutely offer the best exterior ballistics, they are great flyers. The trade off of aerodynamics is the damage they do to the target, which is simply not as much. In fact, as I've posted many times, the pressure wave following long form factor 6.5mm projectiles is notably smaller and only slightly larger than 5.56mm. Why people think that it will slip through a gas, air, and suddenly become high drag when impacting a target that is largely liquid, is beyond me. Fluids and gases follow the same flow principles. Low drag in flight = low drag in target. Thus you have to drive it at a higher velocity to achieve the same forces due to the lower drag as a higher drag bullet.

It gets lost on people that sometimes, the gains in exterior ballistics doesn't always translate to gains in terminal performance and what follows is a good example. I watched a video on Youtube of a guy shooting hand loaded 6.5 Grendel 123gr SST from a 20" barrel at 500 yards. He achieved 2585 FPS which is pretty good for that cartridge and barrel length, probably about as fast as you can push that load with that barrel length.

He actually shot ballistic gel at 500 yards. Guess what happened? ZERO expansion at all. The 123gr SST bullet just zipped right through with no expansion leaving a pin prick hole no different than 9mm ball ammunition. Assuming the ballistic calculator is even remotely close, impact velocity would be in the neighborhood of 1800 fps, yet we know 120gr SST in 6.8 SPC will still fragment down to about 1700 FPS. Why the difference? It's the same bullet construction from the same company, made from the same materials just in different form factors...drag. What works against 6.8mm bullets when flying through the air actually works for them when impacting the target.

So the key is to balance the trade off between exterior ballistics and terminal performance. There's a sweet spot within a specific velocity range you can achieve from a given case size and barrel length.

Too aerodynamic and you have too little drag in the target on impact, so you don't get adequate or any expansion even though you might impact at a higher velocity and with more energy. If you can't transfer the energy, it doesn't do any good. And the opposite is true of too large of a caliber. If you loose too much energy enroot to the target, you may drop below the expansion / frag threshold even though the larger bullet itself needs less velocity to produce the same forces. There is actually an ideal balance between energy loss rates and drag rates where you have a maximum distance you can achieve expansion / fragmentation of the projectile and it seems 6.8mm is that sweet spot at these velocity ranges.

But getting back to the you tube guy, his conclusion was that even with the hottest hand loads from a 20" barrel using some of the best bullets out there, 6.5 Grendel was not capable of effectively taking game beyond about 400 yards or less depending on the barrel / load. So what advantage does the better exterior ballistics give 6.5 Grendel when hunting game? NONE. It can't utilize it because you've reduced drag too much that you need substantially higher velocity to get any expansion at all vs. 6.8mm and larger calibers.

Here is the video: 6.5 Grendel 500 Yard Ballistics Gel Test | 123gr Hornady SST - YouTube

For reference, I'm not at all mocking or making fun of this guy, he's completely honest and realistic about what the cartridge can and cannot do. It is what it is and claims of it being a long range cartridge aside from paper punching are bogus. For distances beyond 400 yards (hunting) and 500 yards (combat), there are much better choices.

 

8.6: Drag Forces in Fluids - Physics LibreTexts

Take a look at the shapes and their fluid drag coefficients. Notice how the same overall shape extended in length (short square vs. rectangle) have notably different coefficients. This is assuming the same fluid viscosity, same temperature and same velocity of the object.

Now look at the streamlined objects. Their drag coefficients are much lower. So what does this mean in physics terms? Projectiles that are low drag in the air are low drag in the target. You MUST drive them faster to achieve the same resistance forces imparted to the fluid as higher drag objects moving at lower velocities.

6.5mm SST, being smaller diameter AND longer in form must travel at a higher velocity to achieve the same drag forces that cause the nose to expand / fragment as 6.8mm SST at lower velocities. Given than 6.8 typically achieves a higher starting velocity from the muzzle and factoring in that by even 500 yards the velocity difference is still not even 100 FPS, I would NOT expect 6.5 Grendel to offer ANY advantage in range where you can achieve expansion.

And at closer ranges, it offers inferior performance because of the lower muzzle velocity AND lower drag...and this is exactly what we see. Also note, for combat / duty applications, the longer 6.5mm projectiles are simply NOT as stable as shorter fatter 6.8mm projectiles, which is why 6.5mm does not perform consistently against barriers as they tend to destabilize while passing through the barrier and dump their energy too rapidly.

You could use shorter form factor 6.5mm projectiles to achieve similar consistency through barriers as 6.8mm. Like 100gr. But then your ballistic coefficient is in the .350 range, so it looses energy faster than your typical 110gr 6.8mm while achieving only similar velocity to 110gr 6.8mm. Your giving up a lot in one area for the sake of another. It's ironic that 6.8mm in it's shorter form factor is such a great projectile because it's not highly specialized in one area, it's balanced, making it versatile. 7mm outperformed 6.8mm terminally, no surprise, but it was felt that it gave up too much in exterior ballistics. Maybe .300 BO should have been 7mm BO for CQB uses?

The above explanation however is exactly why we have this: "In April of 2006, after a four year, $6 million research project assessing 54 different weapon systems in 8 calibers, the JSWB-IPT wrote:

“The 6.8 mm projectile had a near optimal balance of MASS, VELOCITY, and CONFIGURATION to maintain its effectiveness, even at a lower impact velocity.”

I would take "configuration" to be projectile diameter and length, aka it's overall proportions along with the cartridge dimensions itself which we also know offer superior reliability in an M4 sized chassis. No matter how you slice it, unless your punching paper targets at 800 to 1,000 yards with a 20" barrel off of a bench rest that has nothing do with real world use cases (and at that distance it has the energy of a .22 LR), 6.8 SPC outperforms it's 6.5 Grendel counterpart where it counts at ranges we can actually shoot at under field conditions, weather it is hunting animals or defending against bad guys.

There are much better tools for 1,000 yard shooting or hunting at 500 yards + (which I'd argue is very niche anyway) such as .308, 277 Fury, .270 Winchester, 6.8 Western, 7mm Remington Magnum, .300 Win Mag. 6.5 Gendel is NOT among them and 6.5 Creedmoor also gives up a lot in terminal performance for the sake of exterior ballistics and in my opinion, it should have been 6.8mm Creedmoor.

 

I agree with you . One thing to note is most if not all otm become unstable and yaw when hitting animals or hard targets . I noticed they showed the . 556 bonded but not the 6.5 nor the 6.8 bonded bullets . I'm sure the 6.8 would still do better but both should be compared to 5.56 or it should not have been shown .

Sent from my LM-K920 using Tapatalk

 

To be clear, I'd love to purchase a XM-5 once available (will be years from now at best), however I have my doubts about the added fire power making up for the added weight, ammo capacity and recoil for typical close to medium range applications. I would use that platform to get into long range shooting and building a new skillset.

 

This response is for the original topic of 243 vs 6.8 SPC. (I have decided to start a new thread to cover the police and military aspects) I have not been a huge fan of 6mm for medium-sized animal hunting although improvement in hunting bullets have changed that somewhat. For medium size game at up to 300yards, I prefer the SPC. For smaller game, I prefer the 243 at 150+ yards because its flatter trajectory becomes more important when the target animal is small. For 300 yards and beyond, I don't know, but the flatter trajectory of the 243 makes accurately judging the distance less important. I consider the 6.8 SPC a 400-yard cartridge when using the right ammo only if the distance is known, winds are not an issue, and the shooter has practiced enough to make the shot. I grew up in areas where long shots are relatively uncommon and am one of those people who believes that very, very few hunters are really good enough to make the long shots they brag about.

 

@Nincomp I agree with both of those last 2 posts. They almost mirror my experience and thinking as well. Except for growing up where short shots were the rule, and me thinking that the 6.8 Western is more to my liking than the Fury's hybrid case for reloading.

 

The 6.8 Western has some advantages for long range shooting. It can shoot bullets with longer, sleeker ogives, for example. It requires a longer action (3" vs 2.8") and a longer barrel, but those are not deal breakers for many.