Why did, or will you choose the 6.8?

[VA_Dinger]

"Strengthened" 6.8 bolt for those interested in what exactly has been attempted to strengthen a bolt in the past by others. I believe that they use a different material as well versus a standard AR bolt.

Based on nothing more than reading a few problem threads on another forum................ It seems to have "Issues".

It has be hyped to all hell, but as for being "Advanced" the jury is still out.

[shark31]

I think this would be a good time to interject our one main focus here at M4Carbine.net:

If you are going to post unsubstantiated opinions, they should NEVER create a reason for someone like Bill Alexander, John Noveske, Mark LaRue, or anyone else to have to come here and defend their products or designs.

We all have opinions on gear, and some of them are backed up by real data. Some are not. We are not going to deal with the ones that aren't.

I tried to backup what I said with proof that if you wanted an "enhanced" 6.8 bolt that they were out there as Mr. Alexander said that it was a design flaw he found amusing, and I believe that LMT is working on one for their MRP barrels. The fact is that some of us choose to stay as standard as possible with AR parts as many enhanced products end up being not too terribly advanced. I showed an example for my argument, but yet I get chastised for making claims not backed up by real data when Mr. Alexander makes a reference to steel bars and then says he won't disclose any of his design details regarding to his bolt design. Now who's making voodoo magic claims?

I already said that there were reliability (extractor issues, not strength issues) issues with that bolt, which is why I went with a MP tested PRI bolt. I was just told by Grendolizer that I was making unfounded claims about bolts and wanted to clarify that they weren't unfounded. I believe that you are correct that the design branched from the .499 project, however it is touted as to having several features that would make it stronger and is 6.8 specific. The reliability issues with this design are why I wanted to know more about what Mr. Alexander had done to his bolts to make them "stronger". LW and LMT went into great detail about what they did to "enhance" their new bolt designs so that people could understand what they were buying before they dropped their coin, and I find it difficult to understand why you wouldn't disclose such a thing to the market.

So I don't think that me stating that if the two bolts were designed with the same strengthening features as well out of the same material that the bolt with a thinner bolt face would be inherently weaker. If Mr. Alexander chooses to not disclose what he has done to "enhance" his bolt, then are we just supposed to say "It's stronger than the 6.8 bolt because Bill Alexander says so"? I am not trying to start a 6.? debate, far from it I even reccomended 6.5G to the original poster if it fit his needs, but I don't like people saying that my statements are unfounded and then offer no material to rebuke it. If it's stronger show me how so that myself and others can make their own decisions and maybe buy some of your products with some kind of knowledge about what we are spending our money on.

[Nitrox]

As far as I'm concerned Knights is the only manufacturer to ever make an enhanced bolt. The rest are just pretty pieces to spend extra money on.

[SHIVAN]

<<SNIP>>

What you did is make a claim that the 6.5G bolt was weaker than the 6.8SPC bolt.

Now prove it. If you can not, then the supposition is unacceptable.

We will not have industry professionals, manufacturers or dealers having to defend thier designs to accusatory, or errant posting. You are trying to convey YOUR opinion as a statement of fact and placing the responsibility on others to prove your unsubstantiated rumors incorrect. That is unacceptable.

Also, I see that you are now acutely accusing Bill Alexander of "voodoo magic claims". I believe the site owner's are very serious about culling this type of post/poster/

If you need to discuss the matter further, IM me. If my explanation above needs clarification please contact a staff member.

[shark31]

Roger that, I meant that the boltface had more material on it than the Grendel Boltface, and that if the two bolts were made out of the same material or had whatever designs to improve durability built into them that the 6.8 bolt would always be stronger due to the extra material, was I wrong? What features are you talking about?

I clarified my statement further by saying this in my second post responding to the Grendolizer. I didn't get an answer. He said that the bolt had been made out of better materials and had strengthening features built into the design to improve strength and that the 6.8 didn't which was a design flaw. That was incorrect as I pointed out that at least one company had made production "enhanced strength lifetime bolts".

Myself and many others think along the lines of what Nitrox just stated:
There just isn't a good "enhanced" bolt, they all have bugs it seems. These people would run as standard of a bolt as possible which would leave my previous post as correct if the laws of physics still apply at M4carbine.net. If some kind of "proof" is needed to as to how tough they are here's what LW have to say about it:

"Some info on the Advanced Combat Bolt. The ACB is the result of a systematic front to back dynamic analysis of the standard bolt. Nothing is there that doesn't need to be; it's designed and engineered to outlast the host weapon. It is being evaluated by a number of government and commercial enterprises. The steel is actually EN36 BS970 655M13 or 3312 AISI/SAE. It's a nickel chromium case hardening steel. It gives a very hard surface with a strong core and retains a high degree of toughness. There is enough nickel in the steel to give it the same corrosion resistant properties as stainless steel. This still is primarily used in the manufacture of gearing for mining equipment that comes under great sudden load much like an M4 bolt does. Please note the lugs have proper stress releif I beam cuts. These do just that, relieve the lugs of metallurgic stress and thus preventing fractures forming. They also decrease the bolt face mass reducing unlock time.

When you get the technical drawing for the M4 bolt, you note there are tolerences on it. These tolerences are an acceptable range in which the dimension can measure and still be acceptable from a QI perspective. Some tolerences are extremely critical on the lugs as they can effect headspace. These bolts are held to a much tighter tolerence range as LW wanted the lugs to be run to maximum dimension for strength and integrity as well as very consistent headspace lockup. What we have found are some accurracy benefits with the ACB that would equate to running a tight or match chamber without the disadvantage of the decrease in reliable extraction related to a tight chamber. So out of a standard 5.56 Nato chamber, we are getting results similar to a match chamber. Of course there are other factors involved making a match chamber accurate, and this bolt only addresses one of these, but the biggest seems to be consistent headspace lock up from round to round.

Note the dual extractor spring extractor. Both springs are widened at the base so they are captured in the bolt. The springs themselves are rated for millions of cycles. The springs both contain synthetic buffers eliminating extractor bounce or skip. "

Now that sounds kinda like the "proof" that you were talking about. Fact is I don't want to run anything like that in my rifle, I like it to be as true to the original design as possible. Now maybe I'll try one out down the road and I guess that I'll have to when I run a 6.5 G, but I just don't drink that Kool-Aid right now, but at least I know why LW thinks that their product is worth a damn, because they tell you why, and don't just treat people like they don't understand physics.




I didn't mean to get into a 6.? debate, certaintly not with a sponser of this site. I also however don't enjoy being treated like an ingnorant child. I told the poster why I chose 6.8 because that was what the thread purpose was. I stated also that either cartridge should be considered if one wished to change to a bigger caliber. While both will serve you well as a replacement for 5.56 both will have tradeoffs in reliablity off the bat, these tradeoffs will make one caliber choice better suited to your intended purpose. BUT YOU SHOULD LOOK AT BOTH IF YOUR CONSIDERING EITHER.

6.8 has smaller capacity standard sized magazines as well as some very expensive magazines, had a batch of bad ammo/chamber spec problems caused by one type of cartridge by one company, and loses it's legs fairly quickly with the current downgraded ammo. Those are some issues with the 6.8 SPC that you should consider as well. You also have issues with the Grendel, it's proprietary so ammo and rifles parts (recievers, bolts, barrel extensions) are stuck with those companies supply chains, it has a smaller bore with less effeciency out of extremely short barrels, as well as the afformentioned bolt issue.

I will state this as opinion so that I will not get flamed or offend anyone:
1. If I was going to build a more capable SBR (10-12.5") I would go with 6.8 because of the larger bore size and greater efficiency and greater energy from the muzzle where most of my intended targets with this rifle will be close to. Also I want to run a "standard" MP tested bolt and bolt group as I believe it's the most reliable, so due to the extreme forces involved with the gas system on these rifles, using a thicker bolt face on a standard bolt leads me back to the 6.8. The bottom line with these size rifles is that if you shoot them enough, have spare parts (I haven't broken anything yet, but have spares ready and onboard the rifle )
2. If I was going with a carbine (14.5-16) I would stick with a 5.56 and be fine with it. If you have the need because of hunting restrictions in your area, want to build an effective "Recce", or just a hankering to blow up your target and you had to use 6.? and this is your preffered barrel length, you need to look at the hard facts of both cartridges lined out above. One WILL fit your needs more than the other, just decide what your needs are.
3. If I was going to go over 16", it would be silly to go with the 6.8 for the simple fact that you don't get squat for return on the extra barrel length somewhere in the 'hood of 50FPS. To me the purpose of a 6.? with a barrel over 16" is to reach out there and knock the crap out of something at a pretty good distance. The Grendel has found it's home in an SPR type rifle, it outperforms 6.8, 5.56, 7.62x39, or anything else you care to run through an AR15 at intermediate ranges. While I would preffer a larger cross section to hit my target, I feel this is THE way to go if you need to go past 600M hands down. Also I would worry less about parts breaking on a precision rifle as I will hopefully be far far away when it happens. I will buy one eventually when funds allow.

I can't think of anything else to say, If I don't know your intended purpose, then I can't make a real recommendation, but my above opinions should apply. I have pissed off too many people but I feel as though the poster should be more educated now than he was before. I have given my two shiny red pennys on the subject and will now promptly shut up before I anger the mods or sponsers any further.

[Bill Alexander]

Let's play bolts!

First I am going to let you know that you have been royally screwed with your "PRI" MP bolt. The manufacturer (Not PRI) of that item is also making them for near everyone else, they are bought under sub contact and are actually quite a good bolt. In the absence of a standardised loading outside of the current CIP approval and the consequential absence of of a calibrated HP round (discussed on this forum) please illustrate what possible use magnetic particle inspection of the bolt has. In the absence of the overload condition, MP simply indicates that the bolt is made out of a magnetic ferrous base and that it has some internal corner radii. If that core is now running at +48 HVN you will not know (you will find out rather quickly when it is in the gun) nor will you know if you have geater than 10% carbide stringing on the grain boundries, if the case hardening extends to the correct depth or if the defusion during tempering has overhardened any of the thin critical sections. What you have paid for is a test report that says that the item is not cracked prior to recieving any loading whatsoever. Could just be a good cast iron piece at this stage. Maybe you should not believe something just because someone says so?

So what are the flaws that amuse me so? Well just for starters there is no standardization of the chamber or working pressures. In this absence the bolts are simply cut out versions of the plain 223 commercial offerings. Look at that minor diameter under the lugs, notice something, its a 223 bolt. No work at the base of the extractor groove, standard rad at the back of the lugs, no geometry changes in the bolt face except the diameter and the same material. Good stock stuff for a 223, probably about as cheap as you dare make it without having liability issues. Now let's add a bigger cartridge running at higher pressures. No pressure loads, heat treat from the old .mil prints for the M16 and no one specifying a micrograph standard for structure or the core properties at defined points in the cross section. No disrepect to the company making these things, I could do little better if the whole design brief simply consisted of let's cut a bigger case recess.

It would appear that the very simple illustration of fatigue design was not adequate so I will assume that your background in metallurgy, strain energy/shear energy analysis and fatigue calculation is non existant. In this light let us now look at the bolts from LMT and LW. Karl Lewis is a comenserate designer and is both innovative and skilled. In the work on the LMT bolt (we will ignore the very clever extractor for now) he was specifically addressing the bolt life requirements for the enhanced M4 and had to live with those parameters. The thrust loading imposed on that unit is suprisingly exactly the same as that from the M16 but the timing created a more complex stress in the lugs which can and does fail the two lugs either side of the extractor groove. This is the result of uneven loading from high spots and imposed bending stresses The very visible feature of the LMT bolt is the grooving about the major diameter of the lugs. Note how small this groove is and its position reference to the loaded surface. The design very carefully equalises the stresses imposed on each lug without increasing the compressional loading on the material in contact with the barrel extension. So far the bolt is actually no stronger than a conventional design, just a BIT more flexible. The smart bit that adds strength is the little details. Some you can see such as the radius at the rear of the lugs Close inspection shows that this actually undercuts the bolt body ( See illustration with bar above), some you probably cannot see even though it is straight in front of you. There are an additional two features within the LMT lugs as physical geometry that actually enhance the fatigue life of the bolt. (Name both and describe the mechanism and I will sell you a Grendel of your choosing at factory cost (Karl may not apply)). The last part of the LMT bolt is that the material is indeed different. The specific alloy is selected for its resistance to fatigue loading. This alloy and heat treatment is specific to LMT and I will not give details. Suffice to say I am very impressed.

Turning quickly to the LW bolt, I will assume that you are citing this as a design that enhances the strength. Ooo! look it has a groove about the lugs, it must be stronger! If you have read the above and have studied the LMT design you might just notice that the back of the lugs might now increase the contact stress on the material rather than sharing it. Much as I hate PLW I will not indulge in assasinating the design which now rests with a company that is vocally not associated with him. I have read a great deal about 3312 "stainless steel" alloy that increases the strength. Here is a hint; that alloy is not stainless, and it does not appear on the list of alloys that do well in cold enviroments, here's another hint it was ditched for making 1903's

So finally we get to the Grendel/Beowulf bolts. In the first instance I select an alloy that is well suited to the design. Good fatigue life, very desirable micro structure, and then I define a certified heat treatment route with built in tolerances for the structure and properties so that even if we get to the edges of the tempering or cryogenic treatments the result still falls firmly into specification. Lugs get the maximum allowable radius at the rear and we adjust the little features that you named from the LMT discussion to add strength. Remember that little bit of flex needed to equalize the stress at the contact points without allowing too much movement to increase contact loads. This is where the design must be considered as a whole. With the correct material and recess diameter I can now get the spring needed from the bolt without adding grooves to the top of the lugs. But the absence of grooves must still make it crap! because that is what you were told they do on the internet.

You are wading out into some very complex waters. I am going to tell you that I reverse the sequence of the temper and cryo at the end of the heat treatment for the alloy I use, as the retained austenite in the material is stable and the dimesional gains from this reversal outweigh anything I can get from the properties. Can you ellaborate on this descision and let me know your thoughts about the effect of peening and phosphating on the finished part. Even if I laid bare the alloy, heat treatment, geometry and process drawings could you really form an educated conclusion about the strength of the bolt.

I make my living being able to design and sell the stuff that other people cannot get to work. I have over seventeen years of experience in the field of weapons design, many of these as lead engineer on equipment that you might just see as a rumour in the next 10 years. I carry a security classification that you need to be cleared for just to find out what the level is, but I cannot educate you in materials science and fatigue analysis in a forum on the internet. I really do not care either way if you believe me or not, but it would be nice to have you trust my integrity that I will not sell a sub standard or badly designed weapon. As long as I head engineering at AA we build each rifle like it is the one I will buy myself, no seperate line for mil and civ orders and absolutely everything has to be safe.

Bill Alexander

[SHIVAN]

Yeah!

...because Bill Alexander said so.

Bill thank you for responding, and giving insight into your design process.

I appreciate you taking the time to answer the questions posed.

[STAFF]

shark31, you are 100% entitled to YOUR OPINION. M4C wants members to share their FIRST hand knowledge on gear (not something they read on another site). It appear that many of the things you have stated about the 6.5 has been taken from internet rumor (not first person).

Trade secrets are just that (secret) and if a manufacturer doesn't want to spill the beans on one of their designs then so be it. The companies that have been invited to sponsor M4C were picked for their high quality CS and their ethical busines practices. Their word should be consider golden until PROVEN other wise.

[Bill Alexander]

Sorry, I write slowly and spent so much time writing that I have just caught up with the last few posts.

Shivan. Please do not be too hard on Shark31. This industry is full of people who say they have the greatest design in the universe ever, then you spend those hard earned dollars and get a gun that groups like a shot shell on the five occasions that you manage to unjam it, just before it breaks and you find that the company never answers its phones and does not accept liability for any defects whatsoever. I cannot really blame him for being suspitious.

That write up from LW is rather good. Lots of alloy specs: is a back dynamic analysis sort of sketching the design as you run wildly while looking over your shoulder. I also like the bit about metallurgical stress, sounds grand, wonder why you never see the term in text books on either metallurgy or stress analysis. Anyway its got me fooled almost as well as poor old Nitrox who is conviced that KAC builds those enhanced bolts. Check the springs, if they are angled out the thing is LMT.

Bill Alexander

[Nitrox]

Myself and many others think along the lines of what Nitrox just stated:
There just isn't a good "enhanced" bolt, they all have bugs it seems. These people would run as standard of a bolt as possible which would leave my previous post as correct if the laws of physics still apply at M4carbine.net. If some kind of "proof" is needed to as to how tough they are here's what LW have to say about it:

I did not state that.