6.5 grendel and 6.8 spc twist rates and rifling. id like to learn more


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jason41987
August 23, 2012, 07:51 AM
hey everyone.. this isnt a 6.5mm grendel or 6.8mm SPC debate, and im not here to say one is better than the other, and wouldnt like this to get into a debate...

what im trying to do is understand the rifling patterns and twist rates that these two cartridges best excel at.. and why they do best with these configurations of barrels

first, ill begin with what i know about 6.8mm... it seems most people prefer 1:11 twist, and 5 groove barrels, but 1:12 is also acceptable, as is 3 groove... what effect is 1:10 going to have on this cartrdge, does it not stabilize the lighter 110 grain bullets as well? id like to know why 1:11 is sought after and 1:10 is not

also with 6.8mm is the grooves... i hear the odd groove rifling offers higher velocity out of the barrel... but is this really enough of an increase to make odd-groove rifling a must-have?.. if so and if not, why?

as for 6.5mm grendel, ill admit i dont know much about this besides better ballistics beyond 800 yards... but what sort of rifling patterns and twist rates are best suited for this cartridge, and why?.. it seems with the greater variety of 6.5mm cartridges on the market, arisaka, schoenauer, carcano, 260 remington, creedmor that it is in fact easier to find 6.5mm barrels in a variety of grooves and twist rates, as well as projectiles

so yeah.. im trying to understand how the rifling and twist rates effect these two cartridges to get a better understanding on them both

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Shawn Dodson
August 23, 2012, 10:00 AM
Your questions might be better addressed here:

http://68forums.com/forums/

http://www.65grendel.com/forum/

Good luck!

longdayjake
August 23, 2012, 10:08 AM
The reason 6.8 guys like the 1:11 barrels is because they supposedly reduce pressures so that they can squeeze another 20 fps from the case before failure. Last I checked, they also preferred the 3 groove barrels as well for the same reason but that could have changed. The 6.8 crowd changes what they like about as often as the wind blows. This is not to say that they haven't made improvements to the design, but it is the reason there are a ton of different chamberings, barrel twists, rifling options, etc. Mine is a 1:10 SPCII 5 groove chamber and it got poo pooed by the 6.8 crowd as low tier.

The 6.5 is pretty much standardized in the 1:8 barrel twist. That twist will allow you to shoot the really heavy bullets as well as the really light ones. I bought mine back when they still made 1:9 barrels so I can only shoot up to 130 grain bullets. Thats okay because I have settled on the 100 grain TTSX as my hunting round and I am more than pleased with it. The downside to the Grendel is that most of them are built for long range shooting so they are heavy. I use mine for hunting and the weight gets a little bit tiring. AA makes a lightweight version but it is too pricey if you ask me.

68wj
August 23, 2012, 12:48 PM
The 6.8 crowd is not a homogenous group, but 1/11 seems to be settled on. I wouldn't turn down a 1/10 barrel, and there is a 1/7 that was recently released that shows promise in stabilizing 200gr subs and still shooting the lighter bullets in full power loads. My first barrel was 1/11, 4-groove and I also have a 5-groove that I like a lot.

The tweaking done with 6.8 ammo and gun components started almost immediately after the round came out and users found the performance fell short of predictions. Some early adopters saw greater potential than what the field data was showing and started messing around. Several of them got together and did some testing with various barrels and pressure testing equipment. Though there were some weaknesses in the scientific validity of the tests, the take away was that the barrels with slower twists still accurately stabilized the bullets but produced less pressure with the same ammunition. That showed true for lengthening the leade in the chamber too (SPCII chamber). The changes were subtle, but started raising the bar.

Part of the reason the 1/11, 5-groove barrels are brought up so much is mainly because of AR Performance (ARP). He designed his barrels from the ground up, paying attention to land/groove ratio, all the dimensions of the chamber, using good barrel steel and a nitrocarborizing process, and a few extra QC checks. He went through a few generations of products, but his customers are pretty vocal about how well they like the barrels and his reputation for service is solid. Anyone asking for suggestions will hear "ARP" and "1/11, 5-groove" very quickly.

Other 6.8 barrel makers that are held in high regard are also using 1/11 twist. It is fast enough to stabilize the 140gr Bergers, so there is really no reason to go faster unless looking at sub-sonic/long bullet applications.

jason41987
August 23, 2012, 01:10 PM
i like the 6.8mm round, but that doesnt mean 6.5 grendel doesnt have its advantages, easier to find that 1:8 twist you speak of in a variety of twists, and it does share a bolt face with a rather common pre-existing cartridge... was it the trademark and registration of the 6.5 thats essentially killed it as i see 264lbc is taking over that market

jason41987
August 23, 2012, 01:21 PM
all ive been able to find for the 6.8mm barrels as far as blanks for rifles outside of the AR15 platform is 1:11 twist 4 groove from green mountain (not sure if theyre any good) and someone, i forget who says they have a 1:12 with 3 grooves... most common i find though is 1:10 4 groove barrels

i dont want to restrict myself to an AR15 platform as many people do, there are a lot of great cartridges out there i would personally like to see in better platforms but without custom work its impossible

longdayjake
August 23, 2012, 03:05 PM
was it the trademark and registration of the 6.5 thats essentially killed it as i see 264lbc is taking over that market

Well, I saw that happening before the grendel was standardized and accepted by SAAMI. Now anyone can make the barrels.

jason41987
August 23, 2012, 06:04 PM
alexander arms giving up the copyright information in an attempt to save the 6.5 grendel is kind of like trying to un-shoot a deer after youve already eaten it.. if they did it form the beginning when the 6.5 grendel vs 6.8 debates were going on it may have turned out better, because it did have a lot going for it... shared the bolts of a common pre-existing cartridge, plentiful barrel blanks in the twist and grooves they liked, and a large selection of projectiles

helotaxi
August 23, 2012, 11:09 PM
most common i find though is 1:10 4 groove barrelsThose blanks are for the .270 Win, not made with the 6.8 in mind.

i dont want to restrict myself to an AR15 platform as many people do, there are a lot of great cartridges out there i would personally like to see in better platforms but without custom work its impossibleIf you're not wanting an AR, the 6.8 and 6.5 Grendel are terrible cartridges. They are compromises to fit within the restrictions of the AR-15 magazine well. If you're looking at other platforms (unless they use the AR mags) take advantage of what that platform can offer rather than handicapping yourself based on restrictions that don't exist.

jason41987
August 24, 2012, 07:05 AM
im actually looking to convert all my current and future military rifles over to a single cartridge so that i will only need to supply myself with a single supply of ammunition.. many of these are currently 5.56mm rifles, some use AR15 magazines, some do not... i really like the ballistic performance of the 6.8mm... is there something outside of the ballistics that would render this as a poor choice?

i think this sparks a question i should begin in a new thread....

WVHunter1s1k
August 24, 2012, 09:50 PM
Yeah, the Six8er's do like to tweak things. Things do change quickly in the 68 world.:D

Like 68wj said, it deals mainly with land/groove ratio (especially, vs 6-groove) & chamber.
The 4g - 1:11 came out of the fore mentioned tests done in 2008. Which ended up with the SPC II & 4g 1:11 twist being the new standard.

The 1:11 vs 1:10 for 14+" barrels is still debated. Both work fine.

5R & 3-poly are also valued. (BHW 1:11 3 poly & Noveske 1:12 3 poly {Jason's answer})

Oh, that ARP barrel is 1:11.25 - 5R. 'H' from ARP was in on the testing back in 08.

Also, the 6.8 benefits from longer COAL of a bolt. So, it isn't limited to an AR. But, with the different bolt calibers, it does get lost in the mix.
The 6.5 does have better BC on most bullets. Which is part of the long distance appeal of it.
(The 6.8 has the SSA/Berger 140gr for long distances. Some like the 110 Accubond too.)

The 6.8 has plenty bullets to choose from & growing all the time from more manufacture's support. Also, much more loaded ammo to choose from with more coming.


FWIW- Both are great cartridges. You won't loose with either.

jason41987
August 25, 2012, 12:38 PM
so.. for 6.8mm, 16" barrel, if you had a choice between 1:10 6-groove,1:10 4-groove, 1:11 4-groove, or 1:12 3 groove, what would you select?...

and for 6.5mm it seems 1-8" is the standard perfect for those 123 grains everyone loves and for longer bullets as well.. what groove patterns do you prefer for these barrels and why?

longdayjake
August 25, 2012, 02:16 PM
alexander arms giving up the copyright information in an attempt to save the 6.5 grendel is kind of like trying to un-shoot a deer after youve already eaten it..

There was never a need to save the Grendel. Alexander Arms has always had plenty of work on their hands and they have always been back ordered on pretty much everything they produce. Hornady and Lapua make good brass for it. Wolf is supposedly working on a steel case loading for it. These are not signs of a dying cartridge.

and for 6.5mm it seems 1-8" is the standard perfect for those 123 grains everyone loves and for longer bullets as well.. what groove patterns do you prefer for these barrels and why?

There isn't the same obsession with 6.5 and barrel twist/groove pattern as there is with the 6.8. The 6.5 Grendel has always been designed with accuracy in mind so most all the barrels you get will most likely be built for accuracy as the top priority. Part of the reason 6.8 has so many different choices is because it has evolved into a search for maximum velocity. There are subtle things that can be done to a chamber/barrel to help decrease pressure and increase velocity. However, sometimes accuracy can suffer when velocity becomes priority. Because of the longer leade of the 6.8 spcII, it may be harder to find a load that will shoot accurately. That said, you can get most any gun to shoot accurate so long as the barrel is made right. It just takes more work with some chamberings. My experience has been that the 6.8 won't shoot my favorite bullet very well at max velocity. I tried getting the 95 grain ttsx to post good groups with aa2200 but my groups weren't very good. I use the bullet and load anyway when hunting but it can't even come close to my Grendel and the 100 grain TTSX. Velocity wise, the 6.8 is better, but accuracy wise, my Grendel is amazing. So, which is better? Well, I take my 6.8 hunting now because it is about 2lbs lighter. If my Grendel was lighter, I would prefer it.

mshootnit
August 25, 2012, 02:24 PM
jason
I like the armalite factory twist whatever that is for 6.8 mine is real accurate and performs good
68wj, If I am shooting an Armalite 16" spec II and SSA 110 TAC loads, am I leaving a lot on the table not having the 18" and handloads?

68wj
August 25, 2012, 02:53 PM
jason
I like the armalite factory twist whatever that is for 6.8 mine is real accurate and performs good
68wj, If I am shooting an Armalite 16" spec II and SSA 110 TAC loads, am I leaving a lot on the table not having the 18" and handloads?
Not really. I wouldn't worry about the 2". I do like hand loading, but it isn't just about speed.

longdayjake
August 25, 2012, 03:02 PM
Also, SSA tac loads are pretty close to max anyway. You may be able to do better, but it is likely that you will end up ruining brass or losing accuracy. SSA really knows their stuff.

jason41987
August 25, 2012, 03:05 PM
i hear people tell me the ogive of the 6.5mm grendel means it cannot use the same bullets a 6.5x55mm or similar cartridge can use... but isnt the purpose of the shoulder being further back from the bore to allow a longer bullet and the faster rifling used by 6.5 grendel so it will use the same bullets?

i believe the 6.8mm requires a different ogive for shorter bullets so therefor cannot really use the same bullets a .270 winchester would use, and needs the slower 1:11 twist to better stabilize these shorter rounds..

Float Pilot
August 25, 2012, 04:41 PM
The twist rate riquired for a particular bullet is dependent upon the length of the projectile and not the weight. Particularly with the Barnes non-lead bullets.

I started out with a 1 in 10 twist standard barrel for my first 6.8mm and later added a 1 in 11 twist rate Black Hole Weaponry poly rifled barrel.

The 1 in 11 twsit is a 24 inch barrel, so I am obtaining for very nice velocities. However the 1 in 11 will not stabilize some of the longer bullets.

My computer crashed (Babylon toolbar infection) and I have only got it back to an old VGA type ste-up... So I can not post photos right now...

jason41987
August 25, 2012, 06:05 PM
The twist rate riquired for a particular bullet is dependent upon the length of the projectile and not the weight. Particularly with the Barnes non-lead bullets.

I started out with a 1 in 10 twist standard barrel for my first 6.8mm and later added a 1 in 11 twist rate Black Hole Weaponry poly rifled barrel.

The 1 in 11 twsit is a 24 inch barrel, so I am obtaining for very nice velocities. However the 1 in 11 will not stabilize some of the longer bullets.

My computer crashed (Babylon toolbar infection) and I have only got it back to an old VGA type ste-up... So I can not post photos right now...
well, at the same caliber the longer bullet is going to have more weight (usually)

for 6.8mm SPC, i looked at the bullet selections and ballistics for this cartridge.. and i really liked some of the 110-115 grains more than the longer 130s ive read about people being able to use successfully... but the BC increase didnt overcome the lower velocity well enough for me to have much interest in anything longer than 120 grains max

Float Pilot
August 25, 2012, 11:16 PM
well, at the same caliber the longer bullet is going to have more weight (usually)

But not aways.....

A 120 grain 6.5mm Nosler Accubond is longer than a 160 grain Hornady round nose.

A 120 grain 6.8mm (270) Barnes solid banded, is much longer than a 130 grain Sierra boat-tail, or even a 150 grain Speer spitzer.

For example my 1 in 11 poly rifled 6.8m, SPC barrel from Black Hole Weaponry will shoot Speer 150 grain spitzers just fine. In fact it really groups them well.
However the same barrel will not shoot the 120 grain Barnes banded bullets without key-holing. But my 1 in 10 twist barrel will..... Barely....
Too bad since they would be great grizzly skull penetrators.

For a REALLY long 6.8mm bullet take a look at the old Barnes X bulletin 150 grain (SEE PHOTO form my thumb drive) They do not stabilize in the 1 in 11 twist at all... But they would really shred anything within 50 yards as they tumble through.....

helotaxi
August 26, 2012, 06:49 AM
i hear people tell me the ogive of the 6.5mm grendel means it cannot use the same bullets a 6.5x55mm or similar cartridge can use... but isnt the purpose of the shoulder being further back from the bore to allow a longer bullet and the faster rifling used by 6.5 grendel so it will use the same bullets?

i believe the 6.8mm requires a different ogive for shorter bullets so therefor cannot really use the same bullets a .270 winchester would use, and needs the slower 1:11 twist to better stabilize these shorter rounds..Both the 6.5 Grendel and the 6.8SPC use some of the same bullets that were around for their predecessors in their respective calibers but in most cases those bullets were very light and not optimal for the desired use. The 123gn match bullets that are preferred in the Grendel were designed specifically with it in mind to maximize BC and powder capacity within the constraints of the AR-15 magazine length. Numerous bullets have been so designed for the 6.8 with most of them leaning more towards the "performance on living things" end of the spectrum rather than the "punching holes in paper at long range" end, however.

As far as the twist rates go, the Grendel is not known for its stunning velocity and is designed around longer bullets meaning that you need the fast twist to be able to shoot the standard loads. With the 6.8 optimal twist depends. AR mag length or unrestricted (bolt/break action)? Barrel length? Hot-rod loadings? If you are sticking with mag length loads, and a normal barrel length, 1:11 will be fine. If you want hot-rodded loads, then you might start hitting pressure limits a smidgen sooner with a faster twist. If you want to run a short barrel, you might need a faster twist to continue to stabilize the longer bullets (still within mag length) at reduced velocity.

longdayjake
August 26, 2012, 10:50 AM
i hear people tell me the ogive of the 6.5mm grendel means it cannot use the same bullets a 6.5x55mm or similar cartridge can use... but isnt the purpose of the shoulder being further back from the bore to allow a longer bullet and the faster rifling used by 6.5 grendel so it will use the same bullets?


The grendel can use most of the same bullets as the 6.5x55. However, the military round nose bullets aren't really optimal for even the 6.5x55. There are a few that may be difficult but for the most part, the grendel is good from 80 grains up to 140.

The 123gn match bullets that are preferred in the Grendel were designed specifically with it in mind to maximize BC and powder capacity within the constraints of the AR-15 magazine length.

Both the 123 grain Lapua Scenar and the 123 sierra match king were around before the grendel. The only 123 grain bullet I know of that was designed for the grendel was made by Hornady.

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