There have been lots of threads about this lately for some reason so I'll try to explain it.
Formal names first, slang after.
(2004)Remington 6.8x43 SPC-(SAAMI)- has a .050 freebore and when submitted to SAAMI had the correct 45 degree cone angle, has a .278 diameter freebore. Some drawings from PTG has a 80 degree cone angle, no one knows where the error originated.
(Jan 2006)6.8 Remington SPCII-(SPCII)- had a .100 freebore originally drawn by Art of SSA or Barrett not known for sure had the correct 45 degree cone angle, has a .278 diameter freebore, .3085 neck. Some drawings from PTG has a 80 degree cone angle.
(2007)6.8x43 DMR (DMR)- has a .095 freebore, .277 dia and a .305 neck diameter it was designed to correct the bad 80 degree cone angle and be a little more accurate in stainless match barrels because of the shorter freebore and .277 dia leade. Only used in ARP stainless match barrels.
(2008) DMR-C enlarged only enough to take into account the thickness of the chrome in the chamber, neck .0307/.3075-Used only in the chrome lined barrels we had produce in late 2008 and sold in early 2009
(2009) 6.8x43- has a .095 freebore, .2775 dia with the correct 45 degree cone angle and a .3085/.309 neck. A call came in one day which I missed so they called Tim_W, they said they were planning to submit a new drawing to SAAMI with the help of either Rem or Hornady to get the better spec chamber approved by SAAMI. They said they were planning on submitting the SPCII but wanted our input since we had tested the chambers more than anyone. Tim and I talked about what would be best for a true combat chamber, not a hunting or match chamber.
We added a very small amount of taper in the neck and body to aid extraction, the freebore was increased to .2775 from what the DMR chamber had, The neck dia was increased to .3085/.309 taper. We guessed if the mil would adopt the cartridge they would call it the 6.8x43 Nato so we just dropped the "Remington" and "SPC" from the name and Tim sent it back to the person that called noting these would be our recommended changes. Of course nothing happened with SAAMI but that is how the 6.8x43 that I presently use came about.
Noveske mod 1-no real idea, they say it has a .100 freebore.
(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.
IMO all of these chambers except the original "Remington 6.8x43 SPC"(SAAMI) will handle high performance ammo.
These chambers seem to confuse many possibly because the public has been exposed to the chambers and given more info than what has been released about the 5.56 and 308 chambers. There are 14 308 chambers that I know about but most of the public thinks there are only 2 the 308 and the 7.62x51 nato. There are at least 10 223/.556 chambers that I know of, the 6.8 isn't different it is just that there is a lot more info being passed around to the public about it.
Old thread about chambers-http://68forums.com/forums/showthrea...A-and-KoTonics)
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
Twist should be chosen based on the needs to stabilize the bullet that's a given. 11 or 12 twist barrels will stabilize every bullet that can be used in the 6.8 including the 140 Berger but not the subsonic 200s.
This is not related to twist.
If the lands are as wide as the grooves in one barrel and half that width in another barrel do you think that makes any difference at all in what kind of pressures the cartridge creates in the barrel?
If most 10 twist barrels have lands the same width as the gooves and most 11 twist barrels have lands half that wide which are likely to cause less pressure?
What we started saying several years ago was if you wanted to get the most performance from the barrel then use a 11-12 twist because they have lands half as wide and SPCII type chambers and can be loaded to higher velocities.
If someone is shooting standard factory ammo and don't care about reloading to the higher velocities they can shoot what ever barrel they want. It's not like it's going to blow up they just wont be able to load to higher velocities and can't use load data posted by those that have barrels that create less pressure.