6.8 info

Cleaning and Terminal Performance

BREAK IN & CLEANING match grade barrels only
Not needed for chrome-lined or Nitro-carburized barrels.
I should have done this long ago, this is just my opinion, there are many ways to break in a barrel and many work this just happens to be the one that works best for me.
When cleaning match grade barrels always clean from the breech, NEVER use bronze brushes and never drag anything back into the muzzle.
Always use a coated rod with a bore guide in the receiver and chamber and push the nylon brush or patch out of the muzzle, unscrew the patch and pull the coated rod out the breech on the last stroke, no need to push out if doing the short scrub method.
If you pull brushes or patches back into the muzzle the grit will round off the edges of the crown and kill your accuracy which is why you purchased a match barrel to begin with.
Do not mix cleaning products, believe it or not you can over clean a barrel.

Before the very first shot is fired use a short cleaning rod with a jag and patches that fit tight in the bore so it takes 2-3 lbs of force to move through the barrel. Apply Iosso paste on the patch and rub it in so the patch is saturated and work the patch back and forth in the throat only 2-3", make sure the patch comes back into the chamber then apx 2" into the bore. On a stainless barrel make about 50-75 strokes then clean the barrel with a patch damp with alcohol not dripping. After the alcohol run a patch with light oil down the barrel just so the bore isn't squeaky clean. This polishes the reamer marks out of the throat and lessens the initial copper fouling. With a CM barrel make it 100-150 strokes, chromelined 200.
The first 10 shots are a real pain but, it's just one of those things that need to be done. Fire the first shot, clean with a copper solvent 4-5 strokes should be plenty, clean that out with alcohol 2-3 strokes, apply light oil 2 strokes, run a dry patch through 1 time to remove excess oil. Fire shot # 2 and repeat the whole process 10 times for stainless, 20 for CM.

This is a recommended series of firing and cleaning during the break in.
Stainless------------------------------------------ Chrome moly
10 one-shot cycles-clean after every shot--------- 20 - one-shot cycles
5 three-shot cycle-clean after every 3--------------5- three-shot cycles
2 five-shot cycle-clean after every 5--------------- 2 - five-shot cycle

In the beginning while breaking in the barrel(first 100 shots) after you clean with solvents, run a patch with alcohol down the barrel to clean out the solvents then run a patch with light oil just so the barrel isn't a squeaky clean dry metal.
The barrel will not copper foul as much when lightly oiled.
After break in you can clean with solvents if needed then alcohol and leave it for better "cold bore" shots.
While this break in period is going on the barrel will copper foul less and less. When you are through for the day you can use foam to completely clean the barrel then do the regular cleaning process and oil.
At the end of the break in period the barrel should start to turn black and get a good burnished bore. I try to watch for copper fouling by running 1 patch with a copper solvent on it through the bore then wait 2-3 minutes and run a dry one through, if it turns blue copper is present and it should be cleaned well, if it doesn't turn blue then I just run a patch with alcohol after the copper solvent. The key is too keep the copper fouling out of the bore while letting the carbon fill the microscopic scratches. Consistency is the key to accuracy, I had to learn to clean to a certain point not clean to bare metal every time. After I get a good burnished bore I use alcohol only to remove excess carbon, depending on how much I shoot and how far along I am in the barrels life I may make 5 or 10 passes with an alcohol patch and check for copper if my accuracy drops off. If any copper fouling is present I just remove the copper, chase with alcohol and oil the first time after the deep clean then go back to normal shooting till it happens again.

RELOADING
* Sizing brass-use RCBS or other small base dies, they will size the brass apx .002 smaller in diameter than regular dies aiding reliability. SSA small primer brass is the best brass on the market, other brands with large primers will give false high pressure signs.
The 6.8 chamber should be .422 in diameter at the base as required by SAAMI, auto loaders need .004 minimum to work reliably if you buy regular dies and they size the brass to .420 they will more than likely not work in a hot chamber. ---when sizing brass check a few in the chamber before sizing 100 or more
* trim the SSA brass to 1.670". ---1.695 " is absolute max but may cause issues.
* turn the case necks with a Forster outside neck turner for the best accuracy, turn .002-.003 off until it cleans apx 90%
* primers -When reloading for semi auto rifles it is best to use hard cup primers, CCI 41 or magnum primers or Wolf 5.56 or magnum primers, Wolf primers are cheap but, consistent.

--OAL- seat the bullets out as far as the mag room will allow and function, PRI mags allow 2.295-2.305" so 2.300-2.305" is a good length for all bullets except the Hornady 115gr OTM(load to 2.280-2.85) and the Speer 130SP.

Due to liability reasons I can no longer give out load data.

Check the reloading section at http://www.68forums.com/ it is the most complete data anywhere.

Bullet choices and characteristics-

by Doc GKR
Join Date: Jul 2006
Location: Palo Alto, CA
Posts: 830

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Some random thoughts on the terminal performance characteristics of some of the more common projectiles in 6.8 mm:
My comments in red
Barnes 85 gr TSX JHP -- These all copper, lead free bullets bullets offer good expansion and penetration, even out of shorter barrels. They would be useful as both an LE and military general purpose load. This projectile is also a good one for hunting medium game.
Deadly on hogs and deer extra velocity increases effective range

Barnes 110 gr TSX JHP -- These all copper, lead free bullets bullets offer acceptable expansion at higher velocities and deep penetration. They would be useful for hunting medium and tougher game. May over penetrate deer unless driven at high velocity,
deadly on hogs especially on shoulder shots

Hornady 110 gr VMAX PT -- This is a great fragmenting bullet and is perfect for CQB/LE SWAT entry work; it is also a good choice for light game.

Hornady 110 gr OTM (loaded by Hornady) -- This is superb general purpose load and an exceptional projectile for military use. It has better glass performance than the 115 gr OTM.

Hornady 115 gr OTM (loaded by Remington) -- This load has dominated military terminal performance testing in 2002-2006 because of it's early yaw and superb fragmentation, even at reduced impact velocities.

Nosler 110 gr Accubond PT -- These are a good bullet for LE intermediate barrier use, particularly glass, as they offer good expansion and deep penetration. They are also a good choice for hunting medium to larger game when a deep penetrating, non-fragmenting projectile is desired.

Remington 115 gr Core Lokt JSP -- Good expansion and penetration make this an excellent choice for law enforcement use through glass and other intermediate barriers, as well as into vehicles. It is also an excellent choice for hunting medium and larger size game.

Sierra 110 gr Pro Hunter JSP -- This is a good general purpose bullet for law enforcement use and offers acceptable performance against glass and other intermediate barriers. It is also a great hunting load for medium size game. Very accurate bullet, works great with heart/lung shots on deer

Sierra 115 gr OTM w/cannelure -- This second generation SMK offers improved feeding reliability and better terminal performance with earlier yaw and better fragmentation than the first generation 115 gr SMK. It is a good choice for military combat and non-barrier LE use.

Sierra 115 gr OTM w/o cannelure -- This first generation SMK is very accurate, but offers somewhat variable terminal performance and is NOT a good combat, LE, or hunting choice--it is best suited for match target shooting.

Speer 90 gr TNT JHP -- Fragmenting varmint type projectile that will work for CQB/LE SWAT entry work; it is also a reasonable choice for light game. Very accurate bullet, great for P. dog and coyotes when driven at high velocities

Just like making sure your AR15 has a true 5.56 mm chamber and proper barrel twist, it is also critical in 6.8 mm to use the original AMU/"Murray" chamber (also known as SPCII), as well as the superior 3 or 5 groove 1/11 or 1/12 barrel twists--the Remington SAAMI spec chamber is too tight (like a .223 chamber) and the 1/10 6 groove barrels needlessly increase pressures and reduce velocity.

SSA has developed true 6.8 mm AP loads, including a 97.5 gr tungsten penetrator similar to M993/M995.

Note: Most of the 6.8 mm bullets are continuing to upset down to at least 2100 f/s

It is very important to keep in mind that the proper 6.8 mm velocity is 2600 fps +/- 50 fps for 110-115 gr projectiles when fired from a 16" barrel. Government organizations who purchase 6.8 mm should specify in their purchase contracts a minimum acceptable velocity of 2500 fps with an objective velocity of 2600 fps for 16" barrels firing the 110-115 gr projectiles. For duty use, flash suppressed powder, crimped primer, waterproofing, and bullet cannelure should be mandatory requirements.

Some additional information on 6.8 mm is available at: http://www.dtic.mil/ndia/2008Intl/Roberts.pdf

I think this is the first time I have ever seen anyone say that the "Murray" chamber is in fact the same as the SPCII

The following photos of gel test performance were taken from another site.
I tried to keep the ratios correct but photos like these are easily manipulated so do not
use these for accurate comparison without comparing the width of the measuring tape
for the length to width ratio. Most tapes are rarely more than 3/4" so use that to
determine if the ratios are correct. The depth where expansion starts and ends are the
only accurate data IMO.

Barnes 85gr TSX @ apx 3000fps

Speer 90gr TNT @ apx 2610fps

Hornady 110gr Vmax @ apx 2522fps

Barnes 110gr TSX @ apx 2592fps

6.8 Remington SPC

It now seems that an early "Murray" chamber design was sent to Remington and that design was sent to and approved by SAAMI. That design had a short leade(throat) chamber, that combined with Hornady secant ogive bullets stuffed into the lands increased the peak chamber pressures.
Somewhere down the line a mistake on the chamber prints created a sharp 80 degree angle in front of the case neck at the corner of the leade, this sharp edge was shaving copper off of the steep angled bullets. A ring made of small copper shavings fused together from heat and compression would form between the case and throat of the barrel. This copper ring acted like a crimp on the bullet and case neck further increasing peak chamber pressure.
Those 2 issues combined with a twist that was unnecessarily fast and poorly designed barrel specs all added up to a major disappointment in performance.

The SPC II chamber design moved the lands forward to .100" and dropped peak chamber pressure enough that they weren't blowing primers but the bad barrel specs were still there along with the sharp copper shaving edge at the throat.

Many think we should drop the "Remington SPC" name and use only "6.86 X 43mm" on the high performance versions because they feel the old Remington specs are holding back the performance of factory loaded ammo due to liability reasons

6.86 X 43mm DMR

I designed the "DMR" chamber to correct that sharp edge and while I was at it decided to shorten the throat a small amount and make the leade dia .277 to increase accuracy of the cartridge. At this point Pac Nor was one of only a few that made barrels other than 10 twist so I tried a 12 twist 6 groove with the new chamber, it seemed to drop the peak pressure and the overall pressure throughout the barrel. The next step was an 11 twist 4 groove barrel, I had Pac Nor make a new button and a dozen or so barrels. I was thinking less grooves meant more bore area and may reduce overall pressures further, it did but not as much as I had hoped. Pac Nor made a 13 twist 3 groove barrel for me and that one did reduce overall pressure and allowed a much stiffer charge. That change gained apx 80fps across the board.
The Pressure Trace test showed the peak chamber pressure hit when the bullet was apx 3-4" down the bore, that told me by reducing the resistance on the bullet and increasing the area behind the bullet by moving it down the bore easier and quicker it would keep the peak pressure down as well as decrease overall pressure sooner. Resistance is the key and bore area is a large part of that. We chose 5 lands and I designed the button to cut what we thought would give the least resistance while maintaining good accuracy.
The 5 land barrels we are using can produce as much as 100fps more than some 11 twist 4 groove barrels with the same load in the same length barrel. We are currently testing 2 new chamber designs, developing a few new bullets and looking for new high energy powders. I am sure no one ever thought this kind of performance was possible from a cartridge that holds 30gr of powder.

THINKING OUT LOUD

UPDATE-
I was finally able to test a few barrel lengths with the same rifling design and a few different rifling designs against each other with loads from the same batch. I only used 2 bullets that I thought would show the least amount of improvement from longer barrels, the 85gr TSX and the 105gr GS SP.
In short velocities are still increasing with every load all the way to 24" by apx 20-25fps.
I used 16, 20 and 24" barrels all 11 twist 4 groove stainless SAM-R barrels. From 16-20" they gained closer to 25fps then from 20-24" they gained 18-20fps.
Update II- after testing different lengths of barrels I decided to test rifling designs in the same length barrels. The Xtreme 11.25 twist 5R barrels produced apx 100 fps more than some 11 twist 4 groove barrels. I believe heavier bullets with slower powder combinations will show more increase/in but not a great amount.

For hunting, plinking, 3 gun or home defense a 16" barrel is probably all anyone needs, for military use there are other things to consider. During Nam they decided on the 20" M16 with the M193 load for a reason, it hits 3250fps which gives good terminal performance. Then the M855 round was chosen to help defeat armored enemies the military thought they would encounter not that the M193 didn't already defeat most on the market they thought an entire army was about to receive good hard plates and needed the extra performance the M855 offered.
Complaints about the 5.56s terminal performance didn't really come up until the M4 was adopted with a much shorter barrel and slower velocities.
I am not an armor expert but something happens at that 3200fps muzzle velocity threshold with fmj and copper projectiles.
It's not that it needs 3200 to penetrate but it needs 3200+ at the muzzle to allow penetration at a distance.
I hear the AP rounds made by SSA will easily defeat level IV armor at the existing velocities near 2800fps.
In order to keep the chamber pressure to near 54000 -58000 psi for combat rifles I believe the slower twist we are now using with the better rifling designs is an absolute must, then depending on the bullet SSA's old combat loading could actually be bumped up possibly another 100fps and still remain in the 54-58000psi range.
A 16" 6.8 with the correct chamber and rifling design (11-12 twist) using 95gr APs and 95gr fmjs to match loaded to a true 54000psi in that configuration would be a great combat rifle, If they dropped the weight of the bullets to around 85gr like the Barnes TSX then they could push them to 3200fps.