Bulletproof AR15 bolt

I believe the developer of the 6.5 Grendel cartridge, Alexander Arms, has worked out the fine details of tuning the gas system. In fact the blueprint for the chamber and other system parts specific to the Grendel gun are proprietary if only by name. They still get the occassional problem with the bolt as I've delineated in my original post.

trg-s338

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trg-s338
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I see that the case head diameter of the 6.5 Grendel is 0.445". The diameter of the .223 is 0.375". So you do have some force issues to deal with in the bolt with the larger cartridge, assuming equal pressures.

Frankly, I'm skeptical that you're going to improve much on the original Colt item. It may be possible to do so with some expertise in both materials and machining, but Colt probably has it developed pretty far as it is. You want to avoid the temptation to achieve greater strength or wear life through greater hardness of the bolt. That probably would be dangerous.

Is there any possibility of increasing the contact area of the lugs? I'm not familiar with the internal design of the AR-15.

-- Ed Huntress

Reply to
Ed Huntress

----- Original Message ----- From: "Ed Huntress" Newsgroups: rec.crafts.metalworking Sent: Thursday, October 04, 2007 2:26 PM Subject: Re: Bulletproof AR15 bolt

It would require a redesign. the original .54" dia. bolt head has 7 radial lugs relatively evenly spaced (except for the extractor/feed rib gap) that are about .103" wide, .279" long, and .095" high. Total shear area is about

0.201 sq.in. Contact area is a little less than 0.068 sq. in. It locks by rotation into the barrel extension(screwed onto the barrel) So a heftier lock-up would entail a new bolt and extension which there probably is room for in a AR15 upper receiver.

Randy

Reply to
Randal O'Brian

That all sounds like it would be tough to improve what you have. I'll bet you'll find that the military specification on the bolt is pretty demanding. Good luck in your quest.

-- Ed Huntress

Reply to
Ed Huntress

A little checking indicates that the M16/AR15 bolt is a pretty slick piece of metal, as I suspected. It's both case-hardened and shot peened (to SAE AMS S 13165 specification). The technical details on materials and so on are available as mil spec 8448510, but you have to sign a non-disclosure and non-use agreement to get it.

Brownell's sells a Teflon-coated version that meets the spec, and the bolt in the Bushmaster apparently does, too. You may be able to get the material spec from them.

When you see an alloy steel that's case-hardened and shot-peened, you can be fairly sure it's being pushed to limits on both strength and toughness. Trying to get the same properties just by substituting steel grades seems problematic.

BTW, there's an interesting-sounding article on M16 bolt failures that you can get for $30. You can read the abstract for free. Go to

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and search on "Failure analysis of the M16 rifle bolt"

-- Ed Huntress

Reply to
Ed Huntress

Its been a while since I had my hands on a M-16, I remember that the bolt has a lot of lugs and a very short arc obviously. For those that never saw one it looks like a short coarse splined shaft.

What is the length of the mating 'splines' in the barrel extension compared to those of the bolt? If the bolts are failing, it would seem they would have the shorter length.

Wes

Reply to
Wes

You are correct. Which is why the receivers can be made from Aluminum.

The bolt is locked into the barrel at the moment of fireing until the pressure drops enough for the gas piston to unlock the bolt and push it to the rear.

The receiver is just a chunk of metal to keep everything in line, and to put the sights on

Gunner

Reply to
Gunner Asch

Indeed. Bolt face thrust is much less than one supposes due to both surface area (55,000 PSI /bolt face surface. However this thrust is actually carried on the locking lugs..so PSI on the locking lugs is

55000 psi /locking lug bearing surface.

In a typical locking lug failure..do they both crack, or just one?

Keep in mind that the 5.56 round is about the same PSI as the Grendel, IRRC, so locking lug pressures should be the same.

So if both are operating at the same peak pressure...some attention must be paid to duration of that peak pressure. Is a longer peak pressure causing the failure? If they are the same (depends on burning rate of the powder used)

As Bruce said..it would be worth while checking out impulse time /gas port pressure time and sampling size. Some piezo sensors on the gas block with comparisons with both the 5.56 and the Grendal, using loadings in the Grendal that appear to be bolt busters.

Move the sensors down to the locking lug area of the barrel and check not only chamber pressure time/locking lug spikes, but the same on relocking. Its possible that relocking too fast could be an issue. Might need a softer spring and a buffer.

Shrug

Wish I had some time and hardware....

Gunner

Reply to
Gunner Asch

Might be interesting to go with 3 larger locking lugs, or even 4.

Gunner

Reply to
Gunner Asch

Fixed 10 rd magazine. Or a magazine that requires tools to remove.

Then all the evil pistol grips and stuff are ok as long as you have 3 or less.

Straight pistol grip, muzzle brake .

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Im thinking of something slightly different....

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Gunner

Reply to
Gunner Asch

Won't work without redesigning the bolt carrier. the reason for the many locking lugs is to decrease the rotation necessary to unlock.

Given that one of the design criteria for the AR-15 was light weight I would guess that the whole weapon was made as small and light as possible and likely the bolt just won't hold any more.

I did a bit of research on the 6.5 and it is loaded to 42,000 psi while the permissible MAP for the AR15/M16 action is only 45,000 psi which isn't really a high pressure, all things considering.

I don't suppose that the original poster will want to hear it but I suspect that the M-16 is just not the correct action to use for that particular cartridge.

Bruce in Bangkok (brucepaigeATgmailDOTcom)

Reply to
brucedpaige

The bolt lugs are quite short, and in addition, the bolt face is bored out to accept the base of the cartridge to get the extractor onto the rim of the case. The end result is that there isn't a lot of metal holding the lugs onto the bolt. If you have to enlarge the recess for the cartridge without making anything else bigger, you got yourself a problem. The AR-15 occasionally busts a bolt, and boring it out will only make matters worse.

The Grendel is supposed to be a great cartridge, but I really think they should have based the design on the 308 version of the rifle.

Doug White

Reply to
Doug White

45,000 psi????

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All my manuals say 52,000 +/-

Gunner

Reply to
Gunner Asch

On Sun, 07 Oct 2007 10:03:42 -0700, with neither quill nor qualm, Gunner Asch quickly quoth:

I believe they use metric psi in Thailand, Gunner. ;)

-- Ultimately, the only power to which man should aspire is that which he exercises over himself. -- Elie Wiesel

Reply to
Larry Jaques

You may be correct as my "research" was one article about the 6.5 & M-16. However the pressure was specified as "MAP" (which I have never seen before) rather then "CUP " which is the more common measurement. It may be that there is a conversion factor in there somewhere.

It did seem pretty low as I commented.

Bruce in Bangkok (brucepaigeATgmailDOTcom)

Reply to
brucedpaige

Metric pounds? Newtons per square inch? Mini-Average-Pascals? Might be pressures measured ala Brit practice, oiled case with the pressure measurement taken inline with the head. Even measuring radially, it makes a difference if the transducer is located in the chamber area or just ahead of the case mouth. Measurement methods have to be the same or comparisons are apples and oranges.

Bolt failures aren't uncommon with wildcats that use bigger case heads in the AR platform, I believe this is one reason the Coast Guard quit using their near-.50 wildcats they bought for plunking boat engines on drug runners. The other problem is/was special operations guys using their M4s like they were light machine guns, I've seen several articles showing pictures where the barrels softened enough to put bullets out through the sides and the increased bolt velocity from the shorter barrels eventually did for the bolts. They didn't crack at the lugs, but broke at the camming slots. Takes dumping a LOT of ammo through the gun in short time, more than most grunts carry as a basic load. Lewis Machine has an "improved" bolt for those guys, might be worth looking at for the O.P. See Black Rifle II for more info.

Stan

Reply to
stans4

The cartridge was supposedly developed to give the M4 (M16 type) greater lethality via larger bullets, more energy and perhaps longer reach. This was to address the complaints of some in the "sandbox" that say in more than one occasion, the enemy just keeps on coming inspite of multi-hits with the .556 NATO round. In that respect, the design was intended for the M4 rifle just as the 6.8 SPC, another round developed for the M4. These two new cartridges were entered into a study commisioned and tested by the Army but so far have declined to adapt as replacement for the .556. The lack of total reliablity and/or performance is part of the reason I suspect.

Getting back to the bolt, you may be right Bruce in that the M16 format is very limiting particularly in the mag well dimensions and the strength of the lockup mechanism. The use of a more powerful cartridge may indeed be too taxing to the design. Perhaps the only answer lies in the use of heavier .556 bullets say in the vicinity of

75 grains and hotter powders in order to achieve more lethality and power. As per discussion I've had privately with one poster, he argues that one only need look to NRA High Power or the National Match competitons to see the improvements those competetor have achieved. Perhaps the answer lies in their developement to technology.

trg-s338

Reply to
trg-s338

After a Computer crash and the demise of civilization, it was learned Gunner Asch wrote on Sun, 07 Oct 2007

01:44:10 -0700 in rec.crafts.metalworking :

You just want a reason to shoot guns, and have someone else pick up the ammo bill.

Reminds me of conversation some years ago, which went sideways into a discussion on how to set up a series of experiments to test some alleged effective sexual trivia.

"This is not a brouhahahaha! This is Science!"

tschus pyotr

Reply to
pyotr filipivich

After answering Gunner I did some further checking and it seems that pressures are measured in several different ways now-days and there may be several different readings for the same loading.

The oldest method is CUP (Copper Units of Pressure) where chamber pressure is measured by drilling a hole in the barrel and measuring pressure by the amount of crush a copper pellet exhibited. Up until the 1960's these figures were usually referred to as PSI. Since the

1960's they are referred to as PSI CUP.

A second, and more modern, method is to replace the copper pellet with a quartz crystal transducer. These readings are now referred to as PSI.

There exist tables that correlate these different readings. For example:

.222 ANSI CUP = 46000 .222 ANSI PSI = 50000

.308 Win ANSI CUP = 52000 .308 Win ANSI PSI = 60000

However, the pressures I found for the 6.5 were listed in MAP (Maximum Average Pressure) which are usually referenced to CUP. So the 45,000 PSI MAP is very close to Gunner's 52000 PSI which is probably an ANSI PSI measurement.

Bruce in Bangkok (brucepaigeATgmailDOTcom)

Reply to
brucedpaige

On Mon, 08 Oct 2007 16:08:44 +0700, with neither quill nor qualm, snipped-for-privacy@gmail.com quickly quoth:

Interesting.

Couldn't they make up their minds between maximum and average? PSI sure has had some competition. We also have psi, psig, and psia. What would Nick Danger, Third Eye, say about this brouhaha?

-- Ultimately, the only power to which man should aspire is that which he exercises over himself. -- Elie Wiesel

Reply to
Larry Jaques

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