Ruger drops all 6.8 rifles

[constructor]

Twist plays almost no role in pressure. This is why SAAMI only suggests twists, but does not have a maximum twist rate. When the M16 went from the 1:14 to 1:12 to 1:7 - some predicted pressure problems because M193 ammo was already hot on pressure. Those concerns turned out to be unfounded. Likewise, 308 has been compared with 1:12 to 1:8 and there were no pressure/velocity differences. 6.8 does not have special rules.



You mean this: http://bulletin.accurateshooter.com/...gun-nationals/

They got good results with the selection blanks they had in stock for their 30 AS.



No doubt that 6.8 can reach higher velocities than 300 BLK. That is one of its advantages over 300 BLK.



Reducing the throat diameter to the minimum dimensions on the 6.8x43 DMR chamber would be expected to raise pressures more than the short-throat does on the SAAMI chamber.



No, I don't think a 0.277 reamer will cut a 0.2770 hole, but I was always speaking about chamber dimensions, not reamer dimensions.



This is the report:

http://www.border-barrels.com/articl...Consortium.htm

It shows that tighter throats raise pressure, regardless of bullet size.



See this article:

http://www.border-barrels.com/articl...Consortium.htm

Specifically the charts like this:



Notice how with 30 caliber bullets there is a 600 BAR (8700 psi) spread between a 0.310 and a 0.307 throat. So each 0.001 smaller in throat diameter, the pressure went up about 2200 psi. Note that this does not require the bullet to be larger than the throat to be true.

Another problem with tight freebores, especially for semi-autos - is that the bullets that are not perfectly concentric in the brass case will rub on the way in and increase the chance of failure to chamber.



Yes, the SPC-II and 6.8x43 chambers have that feature and it does reduce pressure - about 1000 psi typically (1500 max).



From Art at SSA:


The 0.001 smaller freebore of the 6.8x43 DMR would be expected to raise pressures about 2200 psi - so one could expect it to come out at about 600-1200 psi more pressure than a SAMMI chamber. The 6.8x43 chamber I would expect to come out about -100 to 400 psi ahead of the SAAMI chamber, and about 1100 psi behind the 6.8 SPC-II chamber.



308 grooves are 0.3080 +0.0020 -0.0000. So the groove diameter can be anywhere from 0.3080 to 0.3100 and be in spec.



I have never tried to make a 308 groove diameter that was oversized.

I like that "I would expect" statement...real conclusive there.
You posted on 68 forums that you did send a 308 button design in larger in dia than .308 trying to keep the bore size correct and the button manufacturer called you up and asked if you really wanted to do that.
This whole "design a better spec 6.8 barrel" started because SSAs ammo was blowing primers. They loaded some X-treme copper plated(like a TMJ pistol bullet, not a jacketed bullet) bullets that were swelling in the cone and throat area of the chambers. The SAAMI chambers had a sharp edge that would shave off a copper ring as the swollen bullets moved forward into the barrel. Those shavings would build up just ahead of the case mouth causing a restriction and blow the primers. The story goes that at a large mil test session primers were blowing and Barrett blew his lid. Shortly after, maybe Jan 06 I talked to Art about the issues I had seen in some 6.8 barrels. He said they had started to ream the throat .050 deeper to solve the problem. I picked up a chamber drawing from PTG that showed the chambers had a 80 degree cone angle which made a pretty sharp edge to shave the copper rings. I redesigned that chamber into the DMR which corrected that sharp angle and was a true match chamber. In 07 and 08 I cut about 300 stainless Pac Nor and Hart barrels with that chamber design.
Those barrels were pushing the 85gr Barnes to 3200fps and 110gr bullets to 2800fps with no pressure signs. No other barrel made could do that. Art and I went hog hunting in Texas in 07, Kotonics and my barrels were the only ones that would shoot SSAs combat loads without showing swipes. In 08 5 of us got together again with several different brands of barrels to test them against each other in real world tests. Tim W wrote the results of that test session and published it as the 2008 performance report. We compared real SSA ammo in real 6.8 barrels made by several companies and compared the results. Even an idiot could see the difference between the case conditions and they matched the pressures recorded with the pressure trace system.
In 2009 SSA produced some ammo with .309 diameter necks when the max spec was .306. It caused some major over pressure problems because the bullets were being clamped in the case by the chamber neck but it was an ammo isssue not a chamber issue. It effected me more than others because I wasn't using a sloppy chamber. So in 09 I revised the chamber again to work with SSAs oversized ammo, that is the 6.8x43 chamber I use now. The neck is tapered to match the SSA ammo because their necks are thicker at the bottom than the top. The SPCII chamber is actually smaller than the 6.8x43 at the base of the neck.
You keep posting that thing about the 308 over sized bullets and potential problems and trying to relate that to my 6.8x43 chamber for obvious reasons. What you don't know is the actual size of the chambers so again you are thinking and talking about stuff you have never seen, measured or tested. You're putting out info about 308 barrels and oversized bullets and all kinds of stuff that has nothng to do with the real world 6.8 bullets and barrels.
Going all over the internet to find articles that support your cause doesn't prove anything. I can find articles that say Elvis is alive.
We don't have a few hundred barrels out there we have several thousands being used by people, DEA, Seals Leo and wardens that can say without a doubt they work better than SAAMI barrels and are usually 60-80fps faster than 11 twist 4 groove SPCII barrels. We took over a contract because a big companies SPCII barrels weren't reliable. You, someone that has never tested the 6.8 saying there is only 30fps difference and what were we arguing about for 8 years it's all a guess on your part, real people shooting real 6.8s with real bullets know the difference.
If someone made a SAAMI chambered barrel with my 5R rifling it would be better and show less pressure than a SAAMI barrel with 6 grooves. If someone rechambers a SAAMI barrel to the SPCII chamber it makes a difference. If the chamber and rifling is changed it makes a huge difference. Consumers see it, that is what they want to buy. Ruger wasn't selling it so their sales were slow, if something I have moves slow and something else moves fast I would cut it too.
Business is about turning money the more times you turn it in a year the more proffit you make. There are several companies out there that sell hundreds of 5.56s a day. DPMS says they sell 10,000 rifles a month. If I were them I would not sell anything else if that was the fastest way for me to turn money. It's about making money not which caliber it is.
BTW I don't think you or Art either one know the DMR chamber no longer exist except in the custom match barrels I manufactured in 07 and 08.
I can't figure out why either of you are still talking about it.