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+--- Thread: 6.5 x 40 New AR varient (/showthread.php?tid=7664)
6.5 x 40 New AR varient - rasp65 - 04-05-2014
Check this out: http://www.thefirearmblog.com/blog/2014/04/04/long-range-wildcat-ar-15-6-5-40mm/
Full article here: http://sadefensejournal.com/wp/?p=2414
6.5 x 40 New AR varient - JASmith - 04-05-2014
You can tell by inspection that the 6.5x40 has less volume than the 6.8 SPC. Thus the only way the velocities can get to where they are is with interesting pressures. They have to be well above the Grendel, and are possibly greater than the 58ksi frequently mentioned as the pressure for the 6.8.
Further, the 40mm case length may rule out some long-nosed bullets.
It does, however, have more the look of a classic military cartridge.
6.5 x 40 New AR varient - sneaky one - 04-06-2014
The SIX 5 is also a variant of the 6.8 case.
6.5 x 40 New AR varient - stanc - 04-06-2014
JASmith Wrote:[The 6.5x40] does, however, have more the look of a classic military cartridge.Yup. Overall configuration appears better suited to military use than 6.5 Grendel. Plus, the marginally longer case makes design of a viable metallic link a little more certain.
I found it interesting that the 6.5x40 round in the photo has a cannelured FMJBT bullet. I wonder if Shoffner got Lapua to make a special batch, or if someone else machined the cannelures onto COTS 144gr projectiles.
![[Image: 650x607x65_6.jpg.pagespeed.ic.7kFSfRd24B.jpg]](http://www.thefirearmblog.com/blog/wp-content/uploads/2014/04/650x607x65_6.jpg.pagespeed.ic.7kFSfRd24B.jpg)
6.5 x 40 New AR varient - LRRPF52 - 04-07-2014
I noticed the same oddity, Stan. I'm also noticing that they chose to show the 144gr FMJBT at normal COL, not a 123gr.
I'm losing track of how many wildcats there are off the 6.8 case now. This one requires cutting, forming, and firing.
The SIX 5 is just a size job, but case length limits you to short ogive pills in the AR15 magazine well. Don't let that stop anyone from claiming it "smokes" or "beats" the Grendel in performance. A 120gr SMK with .406 BC was recently used to highlight that the SIX 5 works with the long 120gr bullets. The 120gr SMK is not one of the long ones, and lacks a full numerical value in BC compared to the 123gr pills (.510).
I've also recently been looking at hoop stress in the AR15, as well as bolt thrust. Another claim that I accepted at face value has been discovered to not be supported by the results I found when comparing bolt thrust and hoop stress calculations.
http://lassengunsmithing.com/html/CylinderStress.htm
If you push the pressures over 55,000psi and let 58,850psi be your maximum probably mean with any of these cases that have a diameter of .421" or larger (at the base above extractor groove), the hoop stress gets close to or exceeds 5.56 NATO 70,000psi proof loads. If you push the 6.8 SPC II to the pressures I'm seeing listed in Accurate Powder's tables, like 58,850psi chamber pressure, you've exceeded the hoop stress generated by the SAAMI MAP for the Grendel easily, and you have more bolt thrust than the 6.5 Grendel.
These pressures are what are being advertised as the saving grace for some of these SPC wildcats in the AR15, allowing them to blow the Grendel away, yet case capacity is still the same or less, and 3-5kis is not going to give you the performance advantage you're looking for, while running the ragged edge of what is safe in the AR15.
One cartridge I think took a better approach to this is Hornady's 6mm Hagar, which uses case capacity, not pressure, to get more performance. They increased the SPC's case length to do that, which limits mag-fed options for longer target bullets, but spits the 90gr stuff out very fast.
I've gained a new found respect for why the Grendel was kept "on a leash" at 52,000psi, and the bolt is the least of my worries. Bill has said that for years, but you really get an eye-opener when you run the bolt thrust and hoop stress calcs. If we stay with the AR15 thread tennon diameter on the barrel, then we have to accept a series of constraints. The Grendel overcomes these to a large extent with case length being short enough to fit long projectiles in the AR15 mag well, and push them at moderate velocity inside of safe pressures in the system.
If one wants to remove the constraints of the AR15 barrel extension, thread tennon, and upper receiver ID for the tunnel, one needs to design another rifle, that is if you want to push the pressures like a 5.56 NATO does. I'm becoming more and more interested in lower pressures myself. More throat life, longer critical parts life, larger factors of safety...
The thing I still laugh at is that I'm getting way more performance than I ever thought I would at distance.
6.5 x 40 New AR varient - stanc - 04-07-2014
LRRPF52 Wrote:I noticed the same oddity, Stan. I'm also noticing that they chose to show the 144gr FMJBT at normal COL, not a 123gr.Actually, I have to wonder if that is an error in the photo caption. The bullet ogive and meplat look to me more like the 120gr Norma FMJ than the 144gr Lapua. The Norma bullet has almost a secant ogive and a noticeably flat tip, whereas the Lapua projectile is clearly a tangent ogive and more pointy tip. This was discernible in the specimens I had, as well as in the photo below. (Not that it really matters. It's just a point of curiosity.
)![[Image: 390x263xCapture1-390x263.jpg.pagespeed.i...fjYPWl.jpg]](http://www.thefirearmblog.com/blog/wp-content/uploads/2014/04/390x263xCapture1-390x263.jpg.pagespeed.ic.DY8IfjYPWl.jpg)
6.5 x 40 New AR varient - BluntForceTrauma - 04-07-2014
Either cartridge — 6.5x40 or 6.5 Grendel — would pretty much satisfy what I've been asking for in a military cartridge: compact case, high BC bullet. Of course, all claims would have to be objectively tested. Pretty sure the 6.5x40, as LR said, is not gonna "smoke" the 65G in any respect, particularly when you're looking at the protrusion of the projectile into the smaller diameter case.
Regarding the "superior" case taper that is often trotted out to compare the 6.8 case to the 65G: Do you see it in the photo? It might be there "mathematically," but to the naked eye, there's hardly a dime's worth of difference.
Stan, I'm still not worried about the 65G case in machine-gun links, reference the RPD.
At any rate, the gears of change are slowly grinding toward the correct solution that we've known all along, as more and more "invent" what was obvious to The Horde eleven years ago.
John
6.5 x 40 New AR varient - cory - 04-07-2014
I don't know why we keep perpetuating this rumor that there is some problem with getting links for the Grendel case. There's a link somewhere in this section of the forum to another forum where a guy used, the rpd links I believe, and he states it was like the links were made for the Grendel case.
What I see here is the Grendel, a mild case that is limited by the platform, compared to a 6.5 case that's being hot rodded. With the new DPMS GII platform or upcoming Desert Tech DMR, it would be a relatively simple process to machine a bolt to the .308 external dimensions with a bolt face to accept a Grendel case.
Until we see substantially more data on this new 6.5 cartridge, I'm going to be extremely skeptical of it.
JASmith has made valid points to the potential of a problem with the current 6.5mm projectiles against body armor. However, we've design projectiles and loads for the 5.56 NATO, 6.8 SPC II, and 7.62 NATO that can defeat body armor. I find it hard to believe that a projectile in the 95gr-107gr range with the right load couldn't be developed that would sufficiently meet this requirement, especially if we could operate the Grendel at 58ksi safely.
6.5 x 40 New AR varient - BluntForceTrauma - 04-07-2014
cory Wrote:he states it was like the links were made for the Grendel case.
Which, in effect, they WERE.
cory Wrote:JASmith has made valid points to the potential of a problem with the current 6.5mm projectiles against body armor.
Bill A. long ago tested the 6.5 Grendel's potential against body armor, and the results were VERY encouraging. Doesn't take a rocket scientist to know that high sectional density projectiles are just the ticket for penetration and just happen to be the 65G's forté. Right?
John
6.5 x 40 New AR varient - nincomp - 04-07-2014
As I understand it, the 6.5x40 was specifically designed to replace the 5.56x45. Even if the listed performance is a bit optimistic, it does a pretty good job at that. Notice, however, that all three of its loadings are in or near the transonic region at 600m and all are subsonic by 800. Many bullets lose accuracy in the transonic region and become less useful for aimed fire.
As LRRPF52 pointed out, more powder volume is needed to get the desired performance and keep pressures down.
The 6.5 Grendel is much closer to a General Purpose Cartridge (GPC), one which would replace the 5.56x45 AND the 7.62x51 for infantry. When using the Lapua 123, the ballistics are pretty much there.
Unfortunately, the military seems to want lead-free bullets. A bullet made of steel, brass, or a mixture of both will be less dense, so its ballistics coefficient will be lower than a similar shaped copper/lead one. Also, polymer cases are on the horizon. So far, the wall thickness for polymer cased is thicker than brass, meaning less powder. There will also need to be a tracer version.
Now I have bumbled into topics being discussed in the Military & Law enforcement threads, so I will stop.
6.5 x 40 New AR varient - stanc - 04-07-2014
cory Wrote:I don't know why we keep perpetuating this rumor that there is some problem with getting links for the Grendel case.It is not a rumor. It's a verifiable fact that the (5.56mm) M27 link, although its loop diameter can be easily increased to fit the fatter Grendel case, does not allow sufficient case protrusion from the front link loop for proper interface with the front feed pawl and feed tray cartridge stop of a belt-fed machine gun. On the old Grendel forum, I posted a photo that illustrated the problem.
No one has yet shown that a viable disintegrating link can actually be designed and developed for the 6.5 Grendel cartridge.
Quote:There's a link somewhere in this section of the forum to another forum where a guy used, the rpd links I believe, and he states it was like the links were made for the Grendel case.Yes, and even before that guy, horde member RangerRick demonstrated the compatibility with RPD belts in the Grendel LMG thread: http://www.65grendel.com/forum/showthread.php?984-Grendel-LMG&p=9998&viewfull=1#post9998
That's fine for the European armies which use non-disintegrating link belts of the RPD type. Unfortunately, it's quite irrelevant to the disintegrating links used by the US military.
6.5 x 40 New AR varient - cory - 04-07-2014
stanc Wrote:It is not a rumor. It's a verifiable fact that the (5.56mm) M27 link, although its loop diameter can be easily increased to fit the fatter Grendel case, does not allow sufficient case protrusion from the front link loop for proper interface with the front feed pawl and feed tray cartridge stop of a belt-fed machine gun. On the old Grendel forum, I posted a photo that illustrated the problem.
No one has yet shown that a viable disintegrating link can actually be designed and developed for the 6.5 Grendel cartridge.
Yes, and even before that guy, horde member RangerRick demonstrated the compatibility with RPD belts in the Grendel LMG thread: http://www.65grendel.com/forum/showthread.php?984-Grendel-LMG&p=9998&viewfull=1#post9998
That's fine for the European armies which use non-disintegrating link belts of the RPD type. Unfortunately, it's quite irrelevant to the disintegrating links used by the US military.
Well my little sister's pants don't fit me. That doesn't mean that I can't go to the store and buy a pair that does. What you're telling me is we need to make RPD links out of a material that will disintegrate, correct? That's a minor ED (Engineering Drawing) detail, that the M&P (Material & Process) team will deal with.
6.5 x 40 New AR varient - stanc - 04-07-2014
cory Wrote:What you're telling me is we need to make RPD links out of a material that will disintegrate, correct?Nope, that's incorrect. RPD links and M27 links are both made of sheet steel. It's the configuration of the links, and how they interface with the RPD and M249 feed systems, which are quite different.
Quote:That's a minor ED (Engineering Drawing) detail, that the M&P (Material & Process) team will deal with.That's very easy to say. In fact, it's exactly the same (unsubstantiated) claim that others here have made over the last several years.
I'm not from Missouri, but I do subscribe to their state motto. Until somebody proves (or at least provides convincing evidence) that it can be done, I'm going to remain skeptical.
6.5 x 40 New AR varient - cory - 04-07-2014
Stan, brother, if you're telling me we're going to continue to use the M249 feed system than we're screwed from the get go. You're talking like the problem is the links. I assure you if the M249 feed system is involved than that is our worst problem.
6.5 x 40 New AR varient - stanc - 04-08-2014
cory Wrote:Stan, brother, if you're telling me we're going to continue to use the M249 feed system than we're screwed from the get go. You're talking like the problem is the links.Cory, the problem is not the links. The problem is the cartridge. The same link design has been used in 5.56mm LMGs beginning as far back as the Stoner 63, and continuing on up to the present day with machine guns like the CETME Ameli and the HK MG4, to name a couple that come readily to mind.
Even the 6mm SAW of the 1970s used the same basic link design (below, right).
![[Image: 6mmSAW.jpg]](http://i11.photobucket.com/albums/a170/kingwuckfit/6mmSAW.jpg)
Notice how there's about an eighth of an inch of the case exposed above the front link loop, before the shoulder narrows down to the neck? That exposed case shoulder is what a machine gun's front feed pawl pushes against to advance the belt. It's also what bumps up against the feed tray cartridge stop, to position the round for chambering. With the Grendel case, there is virtually no case shoulder exposed above the link loop.
The problem isn't just limited to the M249. Any machine gun that is fed using disintegrating links would face the same issues in trying to convert it to 6.5 Grendel.
6.5 x 40 New AR varient - cory - 04-08-2014
The problem with the M249 feed system is its tolerance. This is due to the fact that it's constructed with cheap stamped components. This is fine when the rifle is new; however, with a little wear the rifle quickly becomes increasingly sloppy. The weapon requires an incredible amount of attention and maintenance; otherwise, it's an incredible frustrating platform. To effectively employ the M249 with any reliability you have to know the weapon in your hands better than you know your own hands. The links are of little consequence here.
Stanc do you see that hole in that link. That instantly tells me that there's potential to decrease the width of that portion of the link and still maintain adequate strength. You must realize that Material Science has come light years since the 60's. I'm certain links can be made smaller and stronger at a comparable if not cheaper cost. However, there will be a substantial initial investment in tooling.
This is a relatively simple Engineering task in a very mature science. This task is no different than designing a magazine to reliably feed a new round, i.e. the Grendel. Have such magazines been designed? Yes, most work very well. Can that magazine design be improved upon? Most certainly.
6.5 x 40 New AR varient - stanc - 04-08-2014
cory Wrote:The problem with the M249 feed system is its tolerance. This is due to the fact that it's constructed with cheap stamped components. This is fine when the rifle is new; however, with a little wear the rifle quickly becomes increasingly sloppy. The weapon requires an incredible amount of attention and maintenance; otherwise, it's an incredible frustrating platform. To effectively employ the M249 with any reliability you have to know the weapon in your hands better than you know your own hands. The links are of little consequence here.Of little consequence??? That's just not true. The links are of major importance to a belt-fed weapon. Without them, at best all you have is an exceedingly expensive, overly complex, and unduly heavy, magazine-fed rifle. That's in the specific case of the M249. For others, like the Mk46 and MG4, you wouldn't have even that much. Viable links are absolutely critical.
Quote:Stanc do you see that hole in that link. That instantly tells me that there's potential to decrease the width of that portion of the link and still maintain adequate strength. You must realize that Material Science has come light years since the 60's. I'm certain links can be made smaller and stronger...All of that has been said before. Unfortunately, as with the others before you, all you have provided is speculation and wishful thinking. It is still yet to be proven that the link loops can be made sufficiently narrow, yet have the necessary combination of strength and elasticity.
Quote:This task is no different than designing a magazine to reliably feed a new round, i.e. the Grendel. Have such magazines been designed? Yes, most work very well.Yes, they do. However, the tasks are quite different.
Grendel magazines have the same external dimensions as 5.56 magazines.
In contrast, Grendel links -- if the loops are narrowed as you suggest -- would have substantially different dimensions than 5.56 links. There is no assurance that such an altered design would work.
6.5 x 40 New AR varient - cory - 04-08-2014
stanc Wrote:Of little consequence??? That's just not true. The links are of major importance to a belt-fed weapon. Without them, at best all you have is an exceedingly expensive, overly complex, and unduly heavy, magazine-fed rifle. That's in the specific case of the M249. For others, like the Mk46 and MG4, you wouldn't have even that much. Viable links are absolutely critical.
All of that has been said before. Unfortunately, as with the others before you, all you have provided is speculation and wishful thinking. It is still yet to be proven that the link loops can be made sufficiently narrow, yet have the necessary combination of strength and elasticity.
Yes, they do. However, the tasks are quite different.
Grendel magazines have the same external dimensions as 5.56 magazines.
In contrast, Grendel links -- if the loops are narrowed as you suggest -- would have substantially different dimensions than 5.56 links. There is no assurance that such an altered design would work.
It's a relative comparison. Yes, the links are critical in the functioning. However, when analyzing the problems of the M249 the links are a minor factor compared to the real issues.
If all this has been said before, by no doubt other engineers, than what does that tell you? At this point you're only being ridiculous. It's being stubborn for being stubborns sake. What do you base your assumption on? Mine comes from professional and academic experience. We can design and build computers that fit in the palm of your hand that are orders of magnitude more powerful than the most powerful computers of the 1960's, yet it's hard to believe we can't design and build a disintegrating link for the Grendel case???
6.5 x 40 New AR varient - JASmith - 04-08-2014
Stan, you have been arguing that the links "aren't proven to be doable" for several years -- back to 2010 or 2011 to my direct knowledge and I infer for much longer than that. Every one of us now understands that US has exclusively used links for more than 50 years. That means that we also agree that a link system will need to be developed and proven as part of a serious development program.
We count on you to point out gaps in logic and knowledge deficiencies as discussions move forward. Looking back over the past four years and more of our discussions, I begin to see that one of the best contributions you can make is to post a list of critical issues and your understanding of their development status.
Recognize that others will add to the list and that some will not concur with either your assessment or the larger community judgement of status. That is normal and reasonable. The resulting discussions will help the right folks build a more effective development plan.
The link issue but one item on that that will need to be worked. The question of link 'do-ability' has been beaten to death. It is time to put it on the shelf in favor of issues more critical to what will result in an adoptable squad and platoon level light and medium machine guns. Keep it on your list, however, because developing and proving link system function will be a critical part of a serious development program.
Further, engineers and other folks familiar with this class of work are very much in accord that functional and reliable links can be developed for this cartridge. That engineering consensus tells us that we can put links on the list of things to be worked, but they are not the show-stopper implied by the repeated and overly lengthy debate.
Let's move on and work some of the other issues -- we will count on you reminding the community if link development doesn't show up as part of a comprehensive R&D plan.
6.5 x 40 New AR varient - stanc - 04-08-2014
cory Wrote:It's a relative comparison. Yes, the links are critical in the functioning. However, when analyzing the problems of the M249 the links are a minor factor compared to the real issues.Irrelevant. Any flaws the M249 has are not the issue.
Quote:If all this has been said before, by no doubt other engineers, than what does that tell you?It's been said by other Grendel fans, none of whom have professed any expertise or experience in design and fabrication of machine gun links.
Quote:What do you base your assumption on?Faulty question. I made no assumption.
Quote:We can design and build computers that fit in the palm of your hand that are orders of magnitude more powerful than the most powerful computers of the 1960's, yet it's hard to believe we can't design and build a disintegrating link for the Grendel case???I never said it can't be done. I said only that no one has proven that it can be done.