Barrel life 6.5 CM vs 308 Win

[Chadc]

I am building a 6.5 CM for my daughter. I have the action, bottom metal and stock but I am waiting on the barrel to begin the build. I have been going over reloading tables and something caught my mind…why is the 6.5 CM barrel life so short when compared to the 308 win? From hodgden reloading, 140 grain Hornady ELD with 4350 max load of 40 gr pushes the bullet at roughly 2650 fps with 60,000 cup. The 308 with a Berger 168 gr bullet with a charge of 42 grains of powder is moving at 2700 fps with 61,000 cup. These two loads couldn’t be more similar but the barrel life of the 6.5 CM is 1/4 of the 308. So, what am I missing? Twist, shoulder slope, magical unicorns…

Someone help this dumb guy out…

 

[stello1001]

In case you didn't know, the .308 is much bigger than the .264 of the 6.5. They are burning almost the same amount of powder so it's burning all that much through a small hole in the 6.5 or its burning all tuat much through a large hole. Pretty much how I can sum it up lol.

 

[Makinchips208]

The 308 is blessed by God?

 

[Chadc]

In case you didn't know, the .308 is much bigger than the .264 of the 6.5. They are burning almost the same amount of powder so it's burning all that much through a small hole in the 6.5 or its burning all tuat much through a large hole. Pretty much how I can sum it up lol.

So, all other things being similar, bore diameter is accounting for the reduction of barrel life. We are taking about a little over 1 millimeter in diameter between the two cartridges; doesn’t seem like much.

Thanks for the input.

 

[JerkyFreak]

That "little over 1mm" accounts for around 25% of bore area. It is actually quite a bit.

 

[Chadc]

Just found this from 2013 from Frank at Bartlein Barrels:

“Longer bullets vs. shorter bullets. Regardless of velocity the longer bearing surface of the longer bullet will wear the barrel faster. If you shoot 155gr. .308 bullets at 2900fps. and you also shoot 175gr. bullets at 2700fps. yes the 175's are going a little slower but not enough to make a difference but again the longer surface area will wear the barrel more quickly. No way around it.”

Later, Frank

So, the bullet length is a cause…

 

[DRAKENBURG]

So, all other things being similar, bore diameter is accounting for the reduction of barrel life. We are taking about a little over 1 millimeter in diameter between the two cartridges; doesn’t seem like much.

Thanks for the input.

It is better to think in area instead of diameter. So yes, the 6.5CM is only ≈15% smaller in diameter than the 308, but when you look at area, the 6.5 Creedmoor is ≈27% less than the 308. From 600m onwards, the 6.5CM start to significantly outperform the 308 while having ≈22% less recoil.

So yes, the 308 has twice the barrel life, but also more recoil and inferior external ballistics.

In general there is not such thing as "The best cartridge", there is only "The best cartridge for my specific needs". It is always a battle between Internal, External and Terminal ballistics.

 

[Chadc]

That "little over 1mm" accounts for around 25% of bore area. It is actually quite a bit.

Actually it is 16.67 %…which is accounting for 75% less barrel life. Crazy. Damn, My bad, I thought you were comparing diameter. Bore area is math I cannot conceive at this moment.

 

[vno.helix]

Granted I never finished school but isn't 1.1/7.62 more like 14%?

 

[Chadc]

It is better to think in area instead of diameter. So yes, the 6.5CM is only ≈15% smaller in diameter than the 308, but when you look at area, the 6.5 Creedmoor is ≈27% less than the 308. From 600m onwards, the 6.5CM start to significantly outperform the 308 while having ≈22% less recoil.

So yes, the 308 has twice the barrel life, but also more recoil and inferior external ballistics.

In general there is not such thing as "The best cartridge", there is only "The best cartridge for my specific needs". It is always a battle between Internal, External and Terminal ballistics.

How did you come up with the area percentage? And yes, there is always a trade off. Thanks

 

[Chadc]

Deleted

 

[DRAKENBURG]

I used the approximate bore diameter of each caliber:

6.5CM ≈ Ø6.5mm
308 ≈ Ø7.62mm

Area = πØ^2

Thus;

Area 6.5CM ≈ π*6.5^2 ≈ 132.73mm^2
Area 308 ≈ π*7.62^2 ≈ 182.41mm^2

Thus;

6.5CM area decrease relative to 308 ≈ (Area 308 - Area 6.5CM) / Area 308 ≈ 27.2%
308 area increase relative to 6.5CM ≈ (Area 6.5CM - Area 308) / Area 6.5CM ≈ 37.4%

 

[A.sosa]

.308 shoots well at 4600 rounds fired have complete failure trying to stretch it to 4700. It would not stay on the six foot backer at 1000 yards.

 

[Chadc]

I used the approximate bore diameter of each caliber:

6.5CM ≈ Ø6.5mm
308 ≈ Ø7.62mm

Area = πØ^2

Thus;

Area 6.5CM ≈ π*6.5^2 ≈ 132.73mm^2
Area 308 ≈ π*7.62^2 ≈ 182.41mm^2

Thus;

6.5CM area decrease relative to 308 ≈ (Area 308 - Area 6.5CM) / Area 308 ≈ 27.2%
308 area increase relative to 6.5CM ≈ (Area 6.5CM - Area 308) / Area 6.5CM ≈ 37.4%

Incredible. Thanks.

 

[45-90]

I ran the 308 exclusively many yrs ago for shooting 800 to 1400yds.
Each 308 barrel was used for 8000rds, before changing barrels.
All were hot loads of Varget, varying slightly with new barrel preference.
These barrels were not pampered, but tools heavily used, for purpose.
After 8000 rds the 26" barrel was removed...cut off the chamber area, shortened the barrel to 19" and made an AR 15, chambered it for 30 RAR out of the heavily used 308 bolt barrel.
And it shoots great, as most of the firecracking has be cut out...ready for another 8000rds, on this old barrel.
Pictured a box of 1000 rds of 308 ready to load, have already been loaded and fired.
I shoot more 308s than any other caliber including 223, and 22 LR...9mm is the only contender these days, and may catch up.
I own most of the popular calibers including a couple 6.5 CMs but I have 8 308s 16" to 30" barrels 12, 11.25, 10, 9, 8 twists..all for different purposes. Most likely my favorite caliber.
I would definitely choose one of "my" 308s and handloads, over the 6.5 CM...
If I go smaller the 6 Dasher is my favorite.

 

[Chadc]

I ran the 308 exclusively many yrs ago for shooting 800 to 1400yds.
Each 308 barrel was used for 8000rds, before changing barrels.
All were hot loads of Varget, varying slightly with new barrel preference.
These barrels were not pampered, but tools heavily used, for purpose.
After 8000 rds the 26" barrel was removed...cut off the chamber area, shortened the barrel to 19" and made an AR 15, chambered it for 30 RAR out of the heavily used 308 bolt barrel.
And it shoots great, as most of the firecracking has be cut out...ready for another 8000rds, on this old barrel.
Pictured a box of 1000 rds of 308 ready to load, have already been loaded and fired.
I shoot more 308s than any other caliber including 223, and 22 LR...9mm is the only contender these days, and may catch up.
I own most of the popular calibers including a couple 6.5 CMs but I have 8 308s 16" to 30" barrels 12, 11.25, 10, 9, 8 twists..all for different purposes. Most likely my favorite caliber.
I would definitely choose one of "my" 308s and handloads, over the 6.5 CM...
If I go smaller the 6 Dasher is my favorite.

That’s some serious shooting! I do not know if my daughter will reach a barrel change with this build but I was curious why the 6.5 had a shorter barrel life. Thanks to all for the comments.

 

[Haney]

If you have ever run a cutting torch it will make sense .

 

[JerkyFreak]

I finally retired my first .308 target rifle at 6500 rounds when the grouping went from 1/2MOA to 3/4MOA. It has a tight Palma 95 chamber. Plan is to cut the chamber to SAAMI spec and use on a hunting rifle.

My current target rifle has a long knox specifically to cut off the thread and rechamber again. Should still be good for another 6000 accurate rounds.

 

[spife7980]

Pressure = Force * Area

If pressure stays the same and you decrease the area it drives up the force which erodes the bore quicker.

 

[Mike Casselton]

If you have ever run a cutting torch it will make sense .

^^^^
That right there is exactly what I was going to mention.
For once, I actually decided to read the entire thread before responding.

 

[Turkeytider]

I know that there are variables involved but a hypothetical scenario. Recreational, non-competitive shooter out to 1K yards. Slow shooting. 20-30 rounds a session. Shooting factory, " common " weight match loads. What could one reasonably expect as to approximate barrel life between the 6.5 CM and .308 ? If my club range ever expands to 1K as has been the talk, this will probably move from the hypothetical to the real world as far as a decision is concerned. Thanks.

 

[Makinchips208]

Area = πØ^2

Thus;

Area 6.5CM ≈ π*6.5^2 ≈ 132.73mm^2
Area 308 ≈ π*7.62^2 ≈ 182.41mm^2

That seems incorrect. Im not sure what that upward pointing symbol means though…

The area of a circle is pi times the radius squared

 

[Baron23]

So, all other things being similar, bore diameter is accounting for the reduction of barrel life. We are taking about a little over 1 millimeter in diameter between the two cartridges; doesn’t seem like much.

Thanks for the input.

Proportion of case body diameter to neck ID. Pretty much the same gas funneled down into a smaller bore in the 6.5 accounts for this difference in my limited opinion.

 

[mark5pt56]

Question is, what will the rifle be used for?
May want to look into the 6ARC? If the caliber fit the intended use

 

[Baron23]

That seems incorrect. Im not sure what that upward pointing symbol means though…

The area of a circle is pi times the radius squared

View attachment 8437902

Yes, he used diameter so it should be: A = π (d/2)^2

 

[oldrifleman]

For what ever reason, you are superheating the throat with a little flame thrower and the smaller case is a better flame thrower than the .308. the throat erodes and cracks and with it goes velocity and accuracy. The 6 Creedmore is an even better flamethrower hence the fact it burns out barrels (throats) even faster.

 

[Baron23]

For what ever reason, you are superheating the throat with a little flame thrower and the smaller case is a better flame thrower than the .308. the throat erodes and cracks and with it goes velocity and accuracy. The 6 Creedmore is an even better flamethrower hence the fact it burns out barrels (throats) even faster.

And 25 and 22 CM should be even worse.

 

[Chadc]

If you have ever run a cutting torch it will make sense .

Wow, I just realized in my 56 years, 21 years active duty Army, I have never ran a cutting torch or have ever been asked if I have ran a cutting torch. That is probably why it doesn’t make sense to me.

 

[mark5pt56]

Bottom line is you are taking the same generated pressure and forcing it through a hole, the smaller hole will develop more heat as it takes longer to vent. That's how I understand it anyway.

 

[ilmonster]

You can also take recoil out of the equation if you choose to run a muzzle brake. I recently purchased a Aero Solus in 308, and it had a modest amount of recoil out of the box. Putting on an Area 419 Hellfire Match muzzle brake really cut the recoil. It surprised me. Now, it's super pleasant to shoot! Let us know what you decide...

 

[Huckmeat]

Maybe I'm a curmudgon... Run the shit out of the 6.5, shoot the same load as your other guns, and just keep running it.

If you are lucky enough to have your daughter shoot out a barrel, buy another! It is money and time well spent.

 

[oldrifleman]

Maybe I'm a curmudgon... Run the shit out of the 6.5, shoot the same load as your other guns, and just keep running it.

If you are lucky enough to have your daughter shoot out a barrel, buy another! It is money and time well spent.

I totally agree, barrels are a consumable commodity, my son has a 6.5 Creedmoor, my daughter has 6 Creedmoor. Already planning another barrel for the 6 as she likes her rounds a little on the spicy side (3050-3100 fps).

 

[Chadc]

I totally agree, barrels are a consumable commodity, my son has a 6.5 Creedmoor, my daughter has 6 Creedmoor. Already planning another barrel for the 6 as she likes her rounds a little on the spicy side (3050-3100 fps).

That’s awesome. Yea, I would happily replace a worn barrel.

 

[david walter]

Fact is, only some people, and those people frequent this site, will ever run a 308 barrel to the end of its service life. Ditto 6.5.

Shooting out a 308 is like shooting out a Glock 19. I’ve heard of it, but haven’t actually witnessed it.

 

[FieldGaugeFailure]

Maybe I'm a curmudgon... Run the shit out of the 6.5, shoot the same load as your other guns, and just keep running it.

If you are lucky enough to have your daughter shoot out a barrel, buy another! It is money and time well spent.

This. The cost of the ammo required to smoke that barrel is going to be way more than the barrel itself. I get worried the closer the ratio approaches 1:1.

 

[Rob01]

Fact is, only some people, and those people frequent this site, will ever run a 308 barrel to the end of its service life. Ditto 6.5.

Shooting out a 308 is like shooting out a Glock 19. I’ve heard of it, but haven’t actually witnessed it.

LOL come on over and check out my 700 .308. It's on it's 9th .308 barrel. My Glock 17, 19, 34 and 35 are all still running though.

OP it's as mentioned in the same pressures though a smaller hole. Simple as that. Don't stress it as you can get new barrels.

 

[david walter]

@Rob01 ,

Understood. I meant I haven’t shot one out yet.

But, I’m admittedly a half stepper.

 

[Rob01]

Was just making a joke with you. Now get full steppin'!

 

[Dead Eye Dick]

As always, for better barrel life, find a good accuracy node that is below max. Barrels love it and the $$ stay in the wallet longer.

 

[Cody S]

I don't even mind barrel life on a 6 creed ran hot. lasts me a year if I don't practice.
I think it's about 50 cents per shot of barrel life in a hotroded 6 creed.
it's like, what, 25 cents in a 6.5 creed?
plus bullet pricing differences...
about 15 cents per shot extra is what it costs me to shoot 6 creed, but I really like the trajectories and BC for the 109s and dtacs.
fwiw in my experience, 6 creed gets about 1500 rounds, 25 creed gets about 2k, 6.5 creed gets about 2500ish all numbers are my worthless guesses based on a few bartleins in each chambering.

 

[DRAKENBURG]

That seems incorrect. Im not sure what that upward pointing symbol means though…

The area of a circle is pi times the radius squared

View attachment 8437902

Hi Makinchips208

I was away for a while and only noticed your reply now.

The "^" symbol means, "to the power off".

You are correct, I made an error. I was taught to use the following formula in school:

A = (1/4 )πØ^2

instead of the normal:

A = πr^2

Not sure why.

I mixed up the two formulas which resulted in me using the wrong formula.

Thank you for correcting me.

 

[epeet]

Barrel life also has a lot to do with your expectations. Is the barrel done when the groups double in size? Triple in size?

I used to shoot a lot of 5.45 back when you could pay 7 or 8 cents a round for the stuff. I had a barrel that would keyhole every round past 25m. It looked like a shotgun pattern at 100 yards, maybe 6-8 inches at the end.

I still used it to compete in, and win, local 3 gun matches (max distance about 100 yards). I used it to do a ton of training. It didn't matter that the barrel was shooting big groups at 100, because most of the time I just needed to run steel with it at 50m, or shoot paper at 15-25 yards. It was totally fine for those purposes.

I wasn't aware that 6.5 Creed had 1/4 the barrel life of .308, I thought it had about half the barrel life. And that is a function of having a smaller diameter with the same powder load, as others have explained in this thread. But it is also a function of accuracy expectations by the owners. If you are happy with a 2-3 MOA gun, you might be surprised how long the barrel lasts, for example 4000 rounds? 6000 rounds?. If you are competing in PRS, you may prefer the gun shoots at around .75 MOA, and it will then have a much shorter "useful" life, maybe 1800 - 2200 rounds.

 

[45-90]

.308 shoots well at 4600 rounds fired have complete failure trying to stretch it to 4700. It would not stay on the six foot backer at 1000 yards.

I shot 308 barrels to 8,000 rds many yrs ago, before changing.
And kept good notes on round count.
From notes:
8 -28 -05 7000 rds on 26" 308 barrel.
Shot 4, 3shot groups at 100 yds. Averaged, .239" for the 4 groups.
At 1000 yds windy storm coming in, says on notes.
1000 yds 3 into 3.75" 4, into 8.5" 5, into 9.25" for that endevor.
6-25- 05 6500 rds hit a one gallon juice bottle 3 times at 1400yds.
8 -27- 05 6900 rds on barrel 3 shot group at 1000 yds 2.422"

After 8000 rds I chambered a new blank for 308. It was heavily fire cracked at the chamber
After a few yrs, I took the 700 Rem barrel and turned it down and cut out the 308 chamber, and chambered the 8000 rd barrel for the obsolete 30 RAR, in AR 15 for a cost of a few hundred dollars from cheap parts. Now on its 2nd 8000 rds.
How does it shoot as a 30 RAR after most of the bad area was cut out of the 8000rd 308 barrel?
Very well with the same 155 Lapua it liked when it was a 308 barrel.
I have only 2 6.5 CMs and 8 308s.
The hybrid case is the only reason I still have the 6.5 CMs. Having many decades of experience building and altering 308s I have a pretty good idea what it can do, hybrid cases, to LC cases 16 to 30 inch barrels, 12 to 8 twists, LA, SA, and autos. It's still a contender with excellent performance like 200 gr 715BC SMK at 2856 fps, with Lapua brass or over 2900 with hybrid cases.