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Thread: Ruger drops all 6.8 rifles

  1. #51
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    Quote Originally Posted by rsilvers View Post
    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.
    Last edited by constructor; 01-26-12 at 09:52.

  2. #52
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    Quote Originally Posted by Microalign View Post
    True. I am just wondering if this is just a series of events that is leading to the possible death of the catridge. Will more and more manufacturers stop supporting the cartridge? I suppose it won't matter too much if I reload, but the 6.8 from my perspective seems to be losing popularity rather than gaining. I see even less support for the Grendel.
    We're just a small shop but gaining about $280,000 in sales a year.
    They can't supply enough barrels for us to keep them in stock, we run out before they can get the next batch made and delivered. Hornady said their sales of 6.8 are increasing and they are planning a GMX bullet to add to the line.
    S&B were charging $28 a box for ammo and using the least accurate bullet so who cares if they drop the 6.8. Hornady and SSA will pick up the slack and if Tula, PCP and PPU get into it there will be plenty.
    Just look at all of the bullets that have been added to the .277 line up in the last 4 years. The 6.8 isn't going anywhere but up.
    I think #33 is actually a Sierra 130gr SPBT.

    Available factory ammo



    6.8 SPC
    BVAC--------115gr SMK
    ---------------110 Vmax
    Corbon------110 TTSX
    --------------115 SMK
    --------------115 subsonic
    DoubleTap-----95gr Barnes TTSX
    ---------------110 Vmax
    ---------------115 FMJ
    ---------------110TTSX
    ---------------110 OTM
    Hornady-----110 Vmax
    ---------------110 OTM
    ----------------120SST
    Rem.---------115 FMJ
    ---------------115 HPBT
    --------------115 corelokt
    --------------115SMK

    SSA------------85 TSX------------SSAs loads available in Commercial and Tactical velocities
    ----------------90 TNT
    -----------------95TTSX
    ----------------95 frangable
    ----------------97 AP
    ----------------100 Nosler Accubond
    ----------------110 Nosler Accubond
    ----------------110 Sierra Pro hunter
    ----------------110 TTSX
    ----------------115 SMK
    Wilson Combat---95gr TTSX
    Last edited by constructor; 01-30-12 at 16:20.

  3. #53
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    Ooh!... I designed #28!

    work in progress tho..

    you should see the 762x54 projectile!!
    Last edited by Jack-O; 01-26-12 at 01:21.
    My capacity for self deception is exceeded only by yours.

  4. #54
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    Quote Originally Posted by Jack-O View Post
    Ooh!... I designed #28!

    work in progress tho..

    you should see the 762x54 projectile!!
    You should have run with that 120 back when you sent them to me.
    If you added a small boattail it could become popular. I don't think the Berger 140 will work that well. Crank up a thread on 68forums and see what they think. IMO leave it the same length and just chamfer the bottom, it may end up at 117 or something but look at the photo, it's shorter than the 120SST so better case capacity and longer than the 115s for a better BC.
    Last edited by constructor; 01-26-12 at 01:38.

  5. #55
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    Just a reminder to what COLD stated earlier. Please keep in mind that some of you are addressing people who are very active and working in the industry and may have alot more knowledge than what you read on the internet.



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  6. #56
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    Quote Originally Posted by constructor View Post
    There is no substitute for hands on experience.
    I have no dog in this hunt (other than trying to decide between the two calibers), but this is a truism I have found in many of my scientific endeavors as a researcher. What we "think" and what we "find" are often two very different things whether it is rifle specs or semiconductors.

    A good researcher is also a very good mechanic (and vice versa--good mechanics make good researchers--I learned at least as much from my techs as I did my "mentors")

    Now back to our regularly scheduled programming...

  7. #57
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    I am not writing off the 6.8, but I am cautious to get into it. It appears that PRI mags are of the best quality, and I have been looking at DD's new 6.8 SPCII barrel with a 1:11" twist. I plan on sticking with a 16" barrel. Are any of you guys able to run 130gr bullets out of such a setup?

  8. #58
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    Quote Originally Posted by Microalign View Post
    I am not writing off the 6.8, but I am cautious to get into it. It appears that PRI mags are of the best quality, and I have been looking at DD's new 6.8 SPCII barrel with a 1:11" twist. I plan on sticking with a 16" barrel. Are any of you guys able to run 130gr bullets out of such a setup?
    We tested 130gr Speer and Sierra in 06 they work fine in as slow as a 12 twist, the Noslers are too long to fit in the mag same with the CTs.
    Bunch of load data over on 68forums.com we can get 2450-2500fps out of our 5R barrels using 30gr H335, CCI41 primers 2.3" OAL, work up to that not all barrels will handle it.
    Last edited by constructor; 01-26-12 at 10:42.

  9. #59
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    Quote Originally Posted by constructor View Post
    You should have run with that 120 back when you sent them to me.
    If you added a small boattail it could become popular. I don't think the Berger 140 will work that well. Crank up a thread on 68forums and see what they think. IMO leave it the same length and just chamfer the bottom, it may end up at 117 or something but look at the photo, it's shorter than the 120SST so better case capacity and longer than the 115s for a better BC.
    yep, planned on reducing the ogive a wee bit and giving it a nice boat tail. BT length will depend more on stability that raw BC I think. if I can get a good stable bullet then BC wont matter as much. that thing is gonna expand like crazy too.

    actually when I saw Hornady's 120gr I thought they had nailed it, but now looking at it again I see there is a little room for some tweaking.

    Still waiting on my brass guy to get his equipment in place then we might have a new brass manufacturer too!! I'll also have another manufacturer or two to add to the list.

    lots of stuff working quietly behind the internets back
    Last edited by Jack-O; 01-26-12 at 11:51.
    My capacity for self deception is exceeded only by yours.

  10. #60
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    Quote Originally Posted by Jack-O View Post
    yep, planned on reducing the ogive a wee bit and giving it a nice boat tail. BT length will depend more on stability that raw BC I think. if I can get a good stable bullet then BC wont matter as much. that thing is gonna expand like crazy too.

    actually when I saw Hornady's 120gr I thought they had nailed it, but now looking at it again I see there is a little room for some tweaking.

    Still waiting on my brass guy to get his equipment in place then we might have a new brass manufacturer too!! I'll also have another manufacturer or two to add to the list,

    lots of stuff working quietly behind the internets back
    I don't like the secant ogives Hornady uses but... their 110hpbt is very accurate. There are a bunch of guys wanting a better long range bullet, we're doing a 20" 5R barrel so keep us informed.
    Last edited by constructor; 01-26-12 at 12:27.

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