View Full Version : In need of 6.8 info
I am strongly considering a new build in 6.8mm, most of the info I've found on 6.8's appears to be dated, like before ammo was really made available. If anyone or several can answer a few questions I'd appreciate it. Im going to build on a multi caliber marked spikes lower and was considering spikes components for the upper as well, the availability seems to be there and the prices seem to be right, though the bcg is $60-$70 more than their 5.56. My question is am I missing out on a manf. With more insight or better quality than spikes in 6.8?
I would like to use this rifle for home defense, target shooting and also really looking to use it for deer. It appears from what I've seen the 6.8 is like the .270 win. but standard bullet weights in 6.8's I've seen are much lower than standard weight .270 any reason for this? Can I safely fire .270 win. In rifle chambered for 6.8mm?
I have seen some different charts on 6.8 velocities and energy but would like some opinions on how it fairs ballistically(will it get the job done) for hunting and defense. Also would I benefit from a longer barrel? Say 18-20 inches, thanks for any info
I have a co-worker who's dad has a 6.8 upper. The co-worker doesn't know anything about the upper other than having just fired the weapon but he did say that they run u.s. g.i. mags loaded with 28 rnds without issue, though I've seen where some say you cannot run 6.8 in mags intended for 5.56.
silviacrazed
08-04-11, 11:14
The 6.8 is a .277 caliber bullet and that's about the only thing it shares with a .270 winchester. The rounds are not interchangeable. The round has come a long way in the last year or so. Look for a spec II chamber with a 1/11 or 1/12 twist. The old SAMMI specs originally released are obsolete. If you want a well built 6.8, look at AR15performance.com I also suggest you go over to 68forums.com and do some reading. I built my first AR this year and after doing some research I went with a 6.8SPC. The round is optimized for shorter barrels, so a 14.5-16in barrel is about all you need. I'm running a 12.5in barrel. The round is extremely effective on deer as well as hogs. Do not use 5.56 mags, there are 6.8 specific mags and they will feed reliably. The bullets are typically lighter bc the larger bullets are longer and can't be properly loaded into a 6.8 case(with enough powder for performance) and still be loaded to a short enough OAL to fit into AR magazines. Some ppl have shot the heavier bullets out of 6.8 bolt guns, but in the AR platform you're limited to the OAL the magazine will allow.
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Thanks a lot, I really appreciate the answers!
most of the info I've found on 6.8's appears to be dated, like before ammo was really made available.
There is a ton of current info here (do a search), and even more at 68forums.com.
There is a ton of current info here (do a search), and even more at 68forums.com.
Definitely go to 68forums.com. lots more info there. There has been lots of change in the 6.8 world in the last few years.
I'm shooting a 12.5. Seems optimal. Need 6.8 magazines, but mine can load 4 or 5 rounds and feed fine in a 20 or 30 round 5.56 mags.
Here are a couple posts when I did some testing of different barrel lengths for 6.8SPC. Since I did the 7" 11" and 16" barrel test new ammo is available that will deliver better numbers (as is evident by the 9" numbers in the second post). Bottom line, the 6.8 makes a great SBR.
http://xcrforum.com/index.php/topic,5007.msg73191.html#msg73191
http://xcrforum.com/index.php/topic,6226.msg92135.html#msg92135
I have been shooting a 6.8 since late 2007, great caliber. I have taken many hogs with my 11" XCR and recently started hunting with a Daniel Defense 6.8, that is a nice rifle.
All of my hogs were between 80 and 180lbs and all went down with one shot.
I would look at DD for their uppers, I don't know if they are selling them separately yet but they make an accurate barrel.
I'm shooting a 12.5. Seems optimal. Need 6.8 magazines, but mine can load 4 or 5 rounds and feed fine in a 20 or 30 round 5.56 mags.
I found some good info here
https://docs.google.com/?tab=mo&authuser=0&pli=1#home
I would also strongly suggest AR15Performance.com. I just made the plunge and ordered a 16" 9130 steel nitro-carburized socom profile barrel and also 9130 bolt for $295 and he just cut them to $275. I have yet to receive it but from what I've read they are the shit. Oh and it's a 6.8x43 chamber which is a huge plus.
Take his 6.8 super bolt as he calls it
http://www.ar15performance.com/inc/sdetail/3255/338
While I wouldn't pay $100 for one, even if you think it's worth only say, $75 then you're getting a barrel for $200 that will supposedly outlast a chrome lined barrel and will be theoretically more accurate than said barrel. He claims they are more accurate than the basic stainless barrels by AR Stoner, Bison, Wilson, and White Oak.
Stag 6.8 bolts can be found for around $60 and are considered acceptable. Add a good AR15 bolt carrier and you shouldn't be paying any more than a 5.56.
ASC (formerly C-Products) magazines can be had for < $20 in a boat load of places.
Just my 3 cents.
Oh and it's a 6.8x43 chamber which is a huge plus.
I am going to disagree on that. That chamber is listed as having a 0.2775 throat. The 6.8 bullet is up to 0.2780.
A tight throat may be ok for certain competition and bench rest rifles, but for a combat style rifle, one should not consider a throat less than the SAAMI spec of 0.2781 because one never wants a situation where it is possible for a bullet to be larger than the throat - or even too close as carbon could be present.
The SAAMI chamber, while it would benefit from some extra lead, has the 0.2781 throat going for it - which will keep pressures down compared to the tighter throat on some attempts at updating the chamber.
If I were making 6.8 rifles, my first choice would be a SPC-II chamber but I would make sure it is a 0.278 or larger throat.
I would take a SAAMI chamber such as in the Ruger over an SPCII or 6.8x43 chamber with a 0.2770 or 0.2775 throat.
As for twist - anything from 1:9 to 1:12 seems good to me. Faster twist does not raise pressure in a significant way, and no one has proven otherwise. There is some evidence that faster twist helps certain bullets have better terminal effects, so I tend to favor faster.
constructor
08-07-11, 12:08
I am going to disagree on that. That chamber is listed as having a 0.2775 throat. The 6.8 bullet is up to 0.2780.
A tight throat may be ok for certain competition and bench rest rifles, but for a combat style rifle, one should not consider a throat less than the SAAMI spec of 0.2781 because one never wants a situation where it is possible for a bullet to be larger than the throat - or even too close as carbon could be present.
The SAAMI chamber, while it would benefit from some extra lead, has the 0.2781 throat going for it - which will keep pressures down compared to the tighter throat on some attempts at updating the chamber.
If I were making 6.8 rifles, my first choice would be a SPC-II chamber but I would make sure it is a 0.278 or larger throat.
I would take a SAAMI chamber such as in the Ruger over an SPCII or 6.8x43 chamber with a 0.2770 or 0.2775 throat.
As for twist - anything from 1:9 to 1:12 seems good to me. Faster twist does not raise pressure in a significant way, and no one has proven otherwise. There is some evidence that faster twist helps certain bullets have better terminal effects, so I tend to favor faster.
Hey Silvers, why don't you tell everyone here exactly how much experience you have with the 6.8 for you to be telling others what they should buy.
An 11-12 twist will handle any length or weight bullet that can be loaded to mag length and shot in a 6.8.
Most bullets are .2765, only Sierra is a full .277 but Silvers may have some inside info about the Remington bullets being oversized at .278.
And last most here are hunters, plinkers and blasters, how many people here reading are buying their own personal rifle to use in combat? Zero.
Yes, I mostly wanted a little heavier round for hunting purpose but you never know about defense. I was intrigued by the 6.8 , I do not want something as heavy as the .308 in the AR platform. I'm weighing my options based in what all of you have shared collectively so thank you
Constructor - if facts are on your side, you don't need to argue Ad hominem.
You are absolutely right about what you said on another forum - that there are also plenty of 223 and 308 rifles with tight throats. Custom 308 bolt action rifle makers do often use 0.3085 freebore to try to get an edge in accuracy. Larger companies who employ engineers and make products for the military generally don't. Accuracy International, Colt, FN, and HK will more likely use normal engineering practice of never designing a female part smaller than an in-spec male part (unless they are going for an interference fit).
Aside from that, I agree that few bullets are over 0.2775. However, a smaller diameter throat will always result in greater pressures than a larger throat. From this paper:
http://www.border-barrels.com/articles/Pressure_Trials_Consortium.htm
It looks like a 0.001 difference is about 2000 psi in 308.
Moreover, even if the bullet is smaller in diameter than the throat, it may still rub due to carbon or not having zero runout in the cartridge. A bullet which rubs can get stuck on extraction, leaving you with a chamber full of powder.
I disagree with your combat-zone comment. I think a lot of us want our rifles designed as if they were for combat.
It seems like the best 6.8 chamber is the Bison 6.8 SPC-II as it has the longer freebore but still has the 0.278 diameter.
constructor
08-07-11, 14:01
Constructor - if facts are on your side, you don't need to argue Ad hominem.
You are absolutely right about what you said on another forum - that there are also plenty of 223 and 308 rifles with tight throats. Custom 308 bolt action rifle makers do often use 0.3085 freebore to try to get an edge in accuracy. Larger companies who employ engineers and make products for the military generally don't. Accuracy International, Colt, FN, and HK will more likely use normal engineering practice of never designing a female part smaller than an in-spec male part (unless they are going for an interference fit).
Aside from that, I agree that few bullets are over 0.2775. However, a smaller diameter throat will always result in greater pressures than a larger throat. From this paper:
http://www.border-barrels.com/articles/Pressure_Trials_Consortium.htm
It looks like a 0.001 difference is about 2000 psi in 308.
Moreover, even if the bullet is smaller in diameter than the throat, it may still rub due to carbon or not having zero runout in the cartridge. A bullet which rubs can get stuck on extraction, leaving you with a chamber full of powder.
I disagree with your combat-zone comment. I think a lot of us want our rifles designed as if they were for combat.
It seems like the best 6.8 chamber is the Bison 6.8 SPC-II as it has the longer freebore but still has the 0.278 diameter.
That chamber was actually designed by Art of Silver State Armory in Jan of 2006 to correct an issue with some copper plated "Extreme" bullets SSA was loading. Some SPCII reamers made by PTG had a mistake with a 80 degree cone angle as early as 2006(shown in the archives on ar15.com in a post by Randall R of "AR15 Barrels"), so some SPCII chambers still have that bad angle others were corrected in early 2010. I received a few stainless barrels from another company in the summer of 2010 for bead-blasting that still had the 80 degree cone angle in them.
The SPCII chamber is like the 5.56 Nato chamber, our 6.8x43 chamber is like the Wylde chamber and the Remington SAAMI chamber is like the .223 chamber in regards to performance and restriction.
Conversation between Art of SSA and Randall about the newly designed SPCII chamber in 2006-
kalwasart [Member]
7/5/2006 12:40:10 PM MDT
Randall, check your 6.8 reamer drawing, at the 45 degree angle at the .2781 lead, is it 1.70185? I have received several drawings from different gun manufactures that have that same dimension. But from another gun manufacture I have 1.7109 which I believe is the correct dimension.
If the 1.70185 is being used that means the 45 degree angle can not exist and you will end up with a sharp corner at the lead into the chamber. The X-Treme combat loads needs this angle otherwise pressure will peak.
I have a 5 inch thick file on testing on the 6.8 and have to come to the following:
1. Neck size does not have anything to do with increasing pressure and I have the computerize test result to prove it.
2. Cartridges has to be made to the low end of the spec with regards to dimensions.
3. Recently found errors in chamber reamer drawings developed a sharp corner leading into the chamber, which is effecting the X-Treme combat loads. In the June issue of SWAT the Combat X-Treme had excellent results out prefoming all others rounds. You can blame the X-treme all you want but it works in some guns and not in others, so you decide, is it the ammo or the weapon? We however made adjustments to the X-treme to work in weapons with chamber issues.
3,. If your weapon can not shoot projectiles at 2,650 FPS than you have a weapons problem.
4. Some guns have very bad timing problems, where the bolt is trying to open while still under high pressure. Cases ejecting from the 1 to 3 o'clock position. The hotter the load the bigger the problem becomes.
5. If the case are blowing primers and expanding above .424 (being max chamber SAAMI) this is direct result of the bolt opening early.
Our triple Shock thru a Barrett and CMMG is getting 2,715 FPS, can your gun handle it?
The Military has tested the X-Treme at velocities of 2,625 FPS and had no problems. But again who's weapon did they test it in.
We have made changes to the X-Treme so it will work with all weapons, mainly down loading it. We are try to offer a good round at a reasonable price unless everyone wants to shoot just the expensive stuff.
If I was a gun manufacture I would be asking myself if a round is working in someone else gun why is it not working in mine? Or the quick answer is Blame the AMMO.
Art - SSA
I agree the 6.8x43 is like the Wylde 223 chamber.
As for the others, there are two things - throat length, and throat diameter.
The NATO bullet can be up to 0.2244 and the throat can be as small as 0.226. That is a 0.0016 difference. The 6.8 SPC-II Bison chamber is 0.0001 larger than the max bullet. So I would not really think of it as a NATO-style chamber, as it is as tight as the Remington SAAMI chamber in throat diameter. The length is comfy though.
constructor
08-07-11, 15:09
From the Border Pressure trials-
It has been found that to achieve good accuracy from any given ammunition, it is usually necessary to use tight throat dimensions and/or barrel dimensions, which may be tighter than the minimum specified by the bodies that regulate cartridge, chamber and barrel dimensions.
Barrels with a bore dimension of .298" and a groove dimension of .3065 are typical of a large number, if not the majority, of barrels used in the sport of Fullbore Target Rifle shooting. Most of these barrels have throat diameters of .3085" or less. Commercially available, C.I.P. approved ammunition could be potentially dangerous in rifles with tight throat and barrel dimensions. Note the bore and throat dimensions are smaller than the .308 bullet size not larger as in the case of the 6.8.
DISCUSSION.
In general, the results behave as expected in that the pressure does increase as the throat and barrel dimensions decrease. The increase in pressures due to barrel dimensions alone is relatively modest - around 200 to 250 bar for each .001" that the barrel dimensions are reduced. This represents a 5% increase in pressure for a 0.3% decrease in barrel diameter. It is interesting to note the sensitivity of pressure to the throat diameter though, particularly when the throat diameter is smaller than the bullet diameter and the bullet is an interference fit into the throat.
Those results were where the bore is .001 smaller than the nominal .308 bullet, not where the throat is apx .005-.001 larger than most 6.8 bullets.
Most 6.8 bullets are .2765 with the Sierra being .277 and the throats in the 6.8x43 are .2775.
This is kind of like saying don't buy a little Mercedes in case your wife gets fat. Mines not fat and fits just fine.
constructor
08-07-11, 15:16
Yes in the SPCII chambers Like Wilson Combat, RRA, Stag, LWRC and Bison most bullets jump .140" before they touch the lands (not before the lands start but before the bullets touch) so there is plenty of room in there.
Those results were where the bore is .001 smaller than the nominal .308 bullet, not where the throat is apx .005-.001 larger than most 6.8 bullets.
If you look on their chart, they have throats which are 0.310. This is right at SAAMI spec. Then they also do 0.3085, which is 0.0005 smaller than max bullet diameter, then they also do 0.307, which is 0.002 smaller than max bullet diameter. The pressures changes ranged from 250 to 700 Bar. That is 3625 psi to 10,150 psi. If you look back at my posts, I only claimed 2000 psi for your 6.8 throat diameter - less than what they found in their test.
Why did I quote less? Because I already compensated for the differences in these two situations.
constructor
08-07-11, 18:13
If you look on their chart, they have throats which are 0.310. This is right at SAAMI spec. Then they also do 0.3085, which is 0.0005 smaller than max bullet diameter, then they also do 0.307, which is 0.002 smaller than max bullet diameter. The pressures changes ranged from 250 to 700 Bar. That is 3625 psi to 10,150 psi. If you look back at my posts, I only claimed 2000 psi for your 6.8 throat diameter - less than what they found in their test.
Why did I quote less? Because I already compensated for the differences in these two situations.
2000psi IF... IF...IF the bullet is .001 larger than the throat of .2775 IF.
So far no one has seen a bullet over .277 and most are .2765" Have you have found some extra large bullets at Remington that you are trying to warn us about? So why do you worry about things that haven't happened and do not exist?
No, that is not correct. Look at the 308 testing where they go from a 0.3100 throat to a 0.3085 throat. Both of those are above the typical bullet diameter of 0.308, and there was about a 2900 psi difference for the 0.0015 drop. Because for 6.8 case there is only a 0.0010 drop rather than 0.0015, I changed the 2900 psi estimate down to 2000 psi.
http://img708.imageshack.us/img708/8202/pressure.png
So even if the bullet is 0.277, and I agree most are, there will still be extra pressure from the smaller diameter throat.
So basically you fixed the short-throat situation of 6.8 SAAMI, and replaced it with a reamer design that found another way to raise pressure probably even more. The solution is the SPC-II Bison chamber, which keeps the original 0.278 diameter throat.
Furthermore, cartridges have runout due to the brass neck-wall straightness - so the tighter the throat diameter, the more likely bullet rub will exist. And it just gets worse with carbon.
constructor
08-08-11, 09:30
No, that is not correct. Look at the 308 testing where they go from a 0.3100 throat to a 0.3085 throat. Both of those are above the typical bullet diameter of 0.308, and there was about a 2900 psi difference for the 0.0015 drop. Because for 6.8 case there is only a 0.0010 drop rather than 0.0015, I changed the 2900 psi estimate down to 2000 psi.
http://img708.imageshack.us/img708/8202/pressure.png
So even if the bullet is 0.277, and I agree most are, there will still be extra pressure from the smaller diameter throat.
So basically you fixed the short-throat situation of 6.8 SAAMI, and replaced it with a reamer design that found another way to raise pressure probably even more. The solution is the SPC-II Bison chamber
, which keeps the original 0.278 diameter throat.
Furthermore, cartridges have runout due to the brass neck-wall straightness - so the tighter the throat diameter, the more likely bullet rub will exist. And it just gets worse with carbon.
Bison did not design the chamber Art of SSA designed it in 2006, catch up man. We have been shooting loads 200fps faster than any factory ammo since 2007 in the 6.8x43 chamber, thousands of barrels with that chamber have been sold, it's more accurate than the SPCII chamber and coupled with the 5R hybrid rifling it is 60-80 fps faster than the 11 twist 4 groove barrels, our 16" barrels are as fast as most 20" barrels on the market. On top of that the 4150 11595 CMV steel with QPQ heat treat will last twice as long as a stainless barrel.
You never did answer me on 68forums, have you ever shot a 6.8 rifle?
So what is your experience with the 6.8 that you can give advice?
From the posts on 68forums it is all theory and NO actual experience.
The end result from the end product is the only thing that matters to the consumer, they don't care why the barrels are more accurate or faster only that they are. Experience and word of mouth about the product sells, theory and inexperience do not.
Bison did not design the chamber Art of SSA designed it in 2006, catch up man.
Please, I was quoting you:
http://68forums.com/forums/showthread.php?18458-6.8-chambers-SAAMI-SPCII-DMR-6.8x43-Noveske-Mod-1
"(2010) New SPCII- Ben with Bison posted a new SPCII drawing that shows a .114 long freebore with .278 diameter and the correct 45 degree cone angle, everything else is the same as the other SPCII."
So what is your experience with the 6.8 that you can give advice?
My facts can only be refuted with facts, not ad hominem attacks. The chart I posted proves that making the throat a smaller diameter, even if the diamater is still larger than the bullet, raises pressures significantly. You are on record saying that the pressure only goes up if the bullet is larger than the freebore. That is incorrect, and so you have no credibility to continue on this issue without at least an apology to say you screwed up.
constructor
08-08-11, 10:04
Ok if the throat is .002 larger than the bullet it makes sense that the high pressure gases produced by the powder that are expanding in order to push the bullet down the bore will go around the bullet and down the bore in front of the bullet and drop pressure in the barrel.
I would rather keep that pressure behind the bullet and get more velocity with the same amount of powder. That could explain why our barrels produce 60-80 fps more than the SPCII chambered 11 twist 4 groove barrels with the same ammo.
Handloads can be loaded to 200fps faster than any factory ammo and shot safely in our barrels so it doesn't seem that the little bit of pressure rise and extra velocity hurts anything.
I sent 5 uppers to a team of 5 Navy guys a few months ago, one called back to say they were getting 3/8" groups at 100yds with Hornady 110gr HPBTs so I think I will live with the slightly larger than bullet diameter throat as oppsed to the huge throat that the SPCII has since our chamber seems to be more accurate and our barrels faster but thanks for pointing it out.
Constructor - I believe your barrels are accurate and can shoot any ammo out there.
Making tighter throats is common for match rifles and Palma shooting, and is a perfectly reasonable thing to do if accuracy is the #1 goal.
You never did answer me on 68forums, have you ever shot a 6.8 rifle?
Yes, I own a Barrett 6.8.
Remember911
08-14-11, 22:56
I have sbr 10.3in barrel gas port .094 using a 9mm buffer runs great HRN 110gr . I allso have a 16in been deer hunting with it for 2yrs love it. all the parts are Stag.
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