Quoted: I plan to build my own monolithic upper using a similar technique that is entirely new to the industry: I have a MI freefloat handguard which is installed on a flat-top upper.  Normally, this would be considered a two-piece design, but I plan to use a special welding adhesive known as J-B Weld to join the upper and handguards together as one, solid piece.  This will also insure that the barrel goes absolutely NOWHERE as the nut cannot move at all.  Pretty innovative, huh?

You are my new God!

What a Novel idea. I'm going straight to my E-Trade account and buy some stock in J-B Corp.

I hope you're going to patent the process???

Also, don't forget to trade mark the name "J-B lith".

I'll be you're first customer!

lord

Quoted: Quoted: I plan to build my own monolithic upper using a similar technique that is entirely new to the industry: I have a MI freefloat handguard which is installed on a flat-top upper.  Normally, this would be considered a two-piece design, but I plan to use a special welding adhesive known as J-B Weld to join the upper and handguards together as one, solid piece.  This will also insure that the barrel goes absolutely NOWHERE as the nut cannot move at all.  Pretty innovative, huh? You are my new God! What a Novel idea. I'm going straight to my E-Trade account and buy some stock in J-B Corp. I hope you're going to patent the process??? Also, don't forget to trade mark the name "J-B lith". I'll be you're first customer! lord

Yes, the J-B lith will have a few limitations, such as the barrel becoming more or less permanently attached, and things like that, but it is a DIY type thing, which cuts thousands, no millions, of hours of lathe and mill time out of the picture.  For those who just HAVE to spend more money for something only marginally better, the VIS is certainly a viable alternative.

Quoted: Yes, the J-B lith will have a few limitations, such as the barrel becoming more or less permanently attached, and things like that, but it is a DIY type thing, which cuts thousands, no millions, of hours of lathe and mill time out of the picture.  For those who just HAVE to spend more money for something only marginally better, the VIS is certainly a viable alternative.

Have you considered working for MGI, no, better yet, ARES?

They need smart minds like yours to succeed in the industry!

Once again, brillant!

Long live the J-B lith!

i can't wait  for the upper to come out it would definitely match my sundevil lower nicely,

You fools need to stop wasting bandwith.  The VIS is a solution to a problem that has been plaguing the military.  Want to know what that is?  Then shut your mouths and get back in your lane.

Quoted: You fools need to stop wasting bandwith.  The VIS is a solution to a problem that has been plaguing the military.  Want to know what that is?  Then shut your mouths and get back in your lane.

Please elaborate O wise one. Please enlighten us to what the military really wants?

lord

Quoted: Quoted: You fools need to stop wasting bandwith.  The VIS is a solution to a problem that has been plaguing the military.  Want to know what that is?  Then shut your mouths and get back in your lane. Please elaborate O wise one. Please enlighten us to what the military really wants? lord

The added stress caused by adding a rail and accesories to an M4 causes the upper reciever to bend.  Sounds cray, but it does.  That in turn puts uneven stress on the bolt, which eventually come up a few lugs short.  Not to mention that monolithic uppers make the mounting of an IR laser more solid.

Yes, you hit it right on the head! Give that man a Cigar!

Yes the forward portion of the receiver flexes enough, no matter the material or process. The LMT MRP and the Vltor VIS answer the problem by spreading the energy through out the upper receiver. The VIS answer the problem of using standard barrel. The boys at Vltor did their homework.

Even though I'm not a fan of the SIR and CASV handguard, they do offer excellent energy spread through out the upper receiver assembly.

I to look forward in playing with a VIS system.

lord

Quoted: Quoted: Quoted: So if it is two parts; is the top rail in the same plane? IE can you take a throwlevered optic and move it forward and retain a similar zero? Or are the two parts just close enough to look monolithic like 60% of the rail systems already on the market? Theoretically if this process can make a solid homogenous part and the machining of the top rail is done post-process it should be no different than an MRP upper. The parts are brazed together and then machined, so in this respect it should be the same as MRP. The MRP is made out of a forging, so it should have added strength compared to the VIS, but other than that and the interchangeable barrels, the systems should perform similarly. Yes... for all intents and purposes, the VIS is one homogenous part... but the technique allows for use of standard barrels, both LMT and Colt have to use a different system as cutting the threads for the barrel nut is next to impossible without starting out with two pieces. The top rail is a continuous rail and the height is SOPMOD spec, using an extruded handguard section has shown to not be an issue as far as strength goes -- Larue and others have been using extrusion handguards for some time with no recorded problems related to the extrusion itself... lighter, cheaper and plenty strong enough. The joining process is what seperates the VIS... well not from any competition, as there really is no one else offering anythinng like it, but expect to see the copies of it hit the street soon. Anyway, other methods were tested and failed -- The VIS did not go from concept to production in a matter or weeks or even months, this has been undergoing design and testing for a long time now.

I see a new 11.5 upper in my future. Thanks for the info man.

Quoted: Quoted: Quoted: Quoted: Larue and others have been using extrusion handguards for some time with no recorded problems related to the extrusion itself... lighter, cheaper and plenty strong enough. I'm pretty sure LaRue rails are machined from billet. I am definately sure they are not... It appears I'm behind in the times. Out of curiosity, how does LMT machine their forgings for MRPs?  Do they encounter the same problems LaRue did machining rails from billet?

Anyone know?

Thanks.

Quoted: Quoted: Quoted: Quoted: Quoted: Larue and others have been using extrusion handguards for some time with no recorded problems related to the extrusion itself... lighter, cheaper and plenty strong enough. I'm pretty sure LaRue rails are machined from billet. I am definately sure they are not... It appears I'm behind in the times. Out of curiosity, how does LMT machine their forgings for MRPs?  Do they encounter the same problems LaRue did machining rails from billet? Anyone know? Thanks.

They are machined from forgings.

Quoted: Anyone know? Thanks.

Here is sort of a monlithic primer... some of this is just a guess on my part and I will try to make those parts known as we go along.

First, LMT and the MRP -- this is machined from a BAF (big ass forging), they are rather proud of this and have pictures of the forging on the web site and often can be seen carrying a raw forging around at shows. The down side to this is that is is VERY machine intensive, I have heard figures that each upper requires several hours of machine time, the real down side to this is the "drop out" rate... the number of forgings that start out to be MRPs, but never make it -- if a bad cut is made three or four hours into the process, the whole thing can be junk.

This causes a couple of issues to the end user, higher cost is obvious... I would guess that LMT is making far less profit on these than most manufacturing models would suggest. The other downfall is that QC has to be relaxed a little, it is not like a loose tolerance will put a $40.00 part in the scrap heap... so they seem to be a little less strict in what is acceptable.

The plus side is that you get a rather robust platform, but add to that the inherent weight of the design and the possibility of specs being out a little. The quick change barrel is in fact a necessity on the design... perhaps a byproduct really. The traditional method of holding the barrel in relies on the front of the receiver being threaded, as actually cutting the proper surfaces and threads literally at the "bottom of a hole" is at best a daunting task, they went with a slip in barrel that is retained by cross/pinch bolts.

The MRP does not have a removable section in the handguard and therefore does not mount a regular 203.

Colt's system is simular in design... but seeing it suggest to me that they are still a ways out on production -- The system shown recently has strong indication that it was "modeled" on a mill. This means that the outer shape was actually cut pass by pass with a small ball end mill that 3D modeled the shape in with each pass, this practice is not uncommon, and when cut at "high resolution" of hundreds of lines per inch can appear to be a forged part. Of course, this process is also very expensive and does not lend well to consistant and fast production.

Colt's answer to mounting the barrel is a bit different... they have the added advantage of a removable bottom section for the GL and this allows them to "hollow" the system out much faster by just hogging material out from the bottom -- I would imagine they use a guide inserted from the bottom to finish the bore and barrel mounting cuts.

The barrel is retained somewhat conventionally, as it has a barrel nut that threads to the front of what would be the receiver and holds the barrel in. The issue here is that cutting the external threads for a conventional (internal threaded, female) barrel nut is again next to impossible, they decided to cut the front of the receiver with internal threads and use an external threaded barrel nut -- a concern here is most likely that the design of the rifle relies on the regular barrel nut to compress the front part of the receiver (the threaded part the accepts the barrel nut) in order to "clamp down" on the barrel... iirc, Colt uses a slightly tapered barrel extension to press lock the barrel in place tight. For this reason, the Colt system does not use a regular barrel assembly.

As stated, there will most likely be some changes to the Colt design as it gets closer to prodcution.

The Vltor system set out to retain the regular barrel assembly and for that reason starts life as two pieces, this allows for the conventional machining at the front of the receiver section and then the integration of both parts into a solid unit... the technique for joining them has been discussed already.

The Larue system... well, at this point seems to be a design, but given that LT already has the extrusion for the handguard, it should not be long before we see it. As I have not seen one, if one exists, this is speculation... but I would imagine that they will esentially use a somewhat conventional upper receiver design and essentially move the block (that holds the forward section of the current handguard) back to the receiver proper and attach the forward section using the same pin and glue technique that has done well for them thus far.

If the receiver section will be specially made for this or not I do not know, but as the threaded section for the barrel nut is wider than parts of the reciever body, it would seem logical that a special part will have to be made ala Vltor's design -- otherwise, one would be left with mounting the section that will support the handguard to the receiver, then mounting the handguard to that, causing two joining processes and there is not much area to work with forward to the ejection port on the starboard side of the gun...

I am sure that somewhere, even as you read this, someone is crafting together another mono upper, so we will see what happens as things start to shake out... The MRP is a fact and in service, the Vltor design is final and will hit the street soon, the LT will most likely beat Colt's system out, just predictions...

Quoted: Here is sort of a monlithic primer...just predictions.

Nice summary and good thoughts.

So...is the VLTOR "reciever" part an extrusion, or a forging?

Just curious...

Good information Gunzilla, thanks for sharing it.  So does LMT just use a long ass end mill to remove material from inside the handguard section of the MRP?  I assume they start off by boring through the reciever end of the upper, correct (basically "gun-drilling" as you referred to it in the post about the LaRue FF)?  Then they would need to remove the excess material from inside the handguard, but because the amount of material that needs to be removed from this area exceeds that needing to be removed from inside the upper, slight mismatches between the original boring cuts could just be machined away.  Is that about right?

Quoted: So...is the VLTOR "reciever" part an extrusion, or a forging? Just curious...

I think the protos are extrusions, I will find out about production pieces...

Quoted: Good information Gunzilla, thanks for sharing it.  So does LMT just use a long ass end mill to remove material from inside the handguard section of the MRP?  I assume they start off by boring through the reciever end of the upper, correct (basically "gun-drilling" as you referred to it in the post about the LaRue FF)?  Then they would need to remove the excess material from inside the handguard, but because the amount of material that needs to be removed from this area exceeds that needing to be removed from inside the upper, slight mismatches between the original boring cuts could just be machined away.  Is that about right?

I don't remember posting about the ealry LT, but yes there was a slight issue on some where the bores did not match up, this did not effect the function however. I have not seen how LMT makes the MRP, if I had to set up a machine though I would drill from both ends, bore and ream the receiver section and insert a guide mandral from the rear that held a hollow shaft boring bar ran in from the front... so yes, any potential for the initial drilling not lining up would be a non-issue.

Quoted: I don't remember posting about the ealry LT...

Sorry about that, I confused you with LordStoner.

Quoted: Quoted: I don't remember posting about the ealry LT... Sorry about that, I confused you with LordStoner.

Was it because of our extreme knowledge or handsome good looks?

Gunzilla,

Great observation and defination.

When will the Colt mono be available? what will the final construction be; casting, forging, billet or extrusion.

If vltor uses extrusions, how will it hold up compared to an forging? If extrusion is weaker, will the vltor design have the same flex issues that the current upper receivers have? from my understanding, their "laminated" design should strengthen the weakest area of the upper receiver. Is that correct?

lord

Quoted: The added stress caused by adding a rail and accesories to an M4 causes the upper reciever to bend.  Sounds cray, but it does.  That in turn puts uneven stress on the bolt, which eventually come up a few lugs short.  Not to mention that monolithic uppers make the mounting of an IR laser more solid.

Urban Myth. If this was true, every heavy profile 20", 24" and 26" match barrel on an AR would be searing bolt lugs from the get-go.
M4 bolt lug failure is the result of using 20" standard rifle bolts in much higher cyclic rate M4 weapons and then exacerbating this by increasing cyclic rate even more by adding a sound suppressor.  When we developed our improved bolt design (E3), I saw this first hand many times as our (KAC) E3 bolt design evolved to a point that it did not break, regardless.  And that testing included a common set of rail mounted accessories along for the ride.
The other "difference" was that we mantained a reasonable (but high) rate of fire.  I know that other "tests" (that went way past our 400 rounds as fast as is possible) may have seen other results, but heating a barrel until it is "plastic," is actually "testing" too many other "combined" variables to blame subsequent failure on a rail system squeezed betweeen the forend cap and the barrel nut.
What M4 Carbines and the 10.5's need are a higher cyclic rate spec bolt, and ammo tailored to burn efficienctly in the shorter barrels.
Nough said...

I love the looks of this design an it looks like a steal at that price. With that I have three questions.

1.) What sights are those on the rifle on page one, especially the front one.

2.) Is that a 10.5 barrel or a 11.5

3.) Will a Gas Piston system work under it. I am looking hard at the L-W System

Okay, so we have two competing theorems:

1) handguards (two-piece or free-floated) that attach at the barrel nut put stress on it and may cause the reciever to flex, thus creating a situation where the bolt might mis-align with the locking lugs and possibly cause breakage of the bolt lugs; therefore, monolithic recievers and handguards that attach on the reciever rail and totally un-load the barrel/reciever interface are the only solution.

2) locking lug failure results from high-cyclic rate weapons and has little or nothing to do with loading the barrel/reciever interface.

*****

Could someone please clarify this?

Quoted: Okay, so we have two competing theorems: 1) handguards (two-piece or free-floated) that attach at the barrel nut put stress on it and may cause the reciever to flex, thus creating a situation where the bolt might mis-align with the locking lugs and possibly cause breakage of the bolt lugs; therefore, monolithic recievers and handguards that attach on the reciever rail and totally un-load the barrel/reciever interface are the only solution. 2) locking lug failure results from high-cyclic rate weapons and has little or nothing to do with loading the barrel/reciever interface. ***** Could someone please clarify this?

Yes, clarification is needed.

From what I've learned and have been told, the barrel is not the issue it's the handguard and the force/leverage it sees. The longer the handguard, the more leverage can be applied with minimal force. The only issue the barrel has is gravity, unless the barrel has contact to the handguard, similar to what the RIS does today. The only way the barrel can affect the alignment is if it was made of tungsten or is four plus feet in length.

Also, isn't that the main reason the RAS 2 was developed? That's my understanding from the KAC crowd.

Rail systems such as the RAS 2, SIR or POF type chassis are a quick fix for the issue at hand.

lord

Quoted: Quoted: Okay, so we have two competing theorems: 1) handguards (two-piece or free-floated) that attach at the barrel nut put stress on it and may cause the reciever to flex, thus creating a situation where the bolt might mis-align with the locking lugs and possibly cause breakage of the bolt lugs; therefore, monolithic recievers and handguards that attach on the reciever rail and totally un-load the barrel/reciever interface are the only solution. 2) locking lug failure results from high-cyclic rate weapons and has little or nothing to do with loading the barrel/reciever interface. ***** Could someone please clarify this? Yes, clarification is needed. From what I've learned and have been told, the barrel is not the issue it's the handguard and the force/leverage it sees. The longer the handguard, the more leverage can be applied with minimal force. The only issue the barrel has is gravity, unless the barrel has contact to the handguard, similar to what the RIS does today. The only way the barrel can affect the alignment is if it was made of tungsten or is four plus feet in length. Also, isn't that the main reason the RAS 2 was developed? That's my understanding from the KAC crowd. Rail systems such as the RAS 2, SIR or POF type chassis are a quick fix for the issue at hand. lord

How is the applying force to the barrel nut different with a railed handguard than with a sling on the end of a barrel? If I pulled a sling tight from the end of the barrel wouldn't it have the same or more leverage than if I applied the same force to the end of a FF tube? Doesn't the barrel transmit more stress to the receiver than a FF tube because the tube deflects more?

ETA: I have never seen one first hand but I have heard of semi-auto rifle with regular handguards sheering bolt lugs.

Thanks, coldblue.

Quoted: Yes, clarification is needed. From what I've learned and have been told, the barrel is not the issue it's the handguard and the force/leverage it sees. The longer the handguard, the more leverage can be applied with minimal force. The only issue the barrel has is gravity, unless the barrel has contact to the handguard, similar to what the RIS does today. The only way the barrel can affect the alignment is if it was made of tungsten or is four plus feet in length. Also, isn't that the main reason the RAS 2 was developed? That's my understanding from the KAC crowd. Rail systems such as the RAS 2, SIR or POF type chassis are a quick fix for the issue at hand. lord

What you are theorizing would only be possible if the bolt locking lugs were attached to the rail forend and not the barrel.   In that way adding weight to the rail could inadvertently misalign the lugs.  At least you agree gravity affects the barrel.  The weight of a match barrel is 10X what a PEQ-2 and a flash light add to a forend, even if all you did was tape them to GI issue plastic handguards.  If you believe that weight out front can misalign barrel extension lugs, look at barrel mass.

RAS-2 was developed to address the only real operational deficiency with the current RIS/RAS, and that is the point of impact shift of the host weapon when the M203 is attached to the barrel via our QD or the former Armorer installed safety-wired bracket.  Same goes for the URX.  The accuracy enhancing potential of Free-Floating a GI issue barrel is a total waste.  You get a lot more for your tax $ by making more accurate ball ammo available than the M855 is.
I have tested this several times, but originally presented this data at the Ft. Benning Block-2 party about 4 years ago.  Simply switching from M855 to Winchester Match 69 Gr. BTHP in a RIS and then a RAS M4 made a significant accuracy improvement vs the RIS/RAS and then FF the same barrels.  Whereas the FF accuracy of those GI barrels with M855 was hardly quantifiable.  

By the way, during those tests the rails were both void of accesories and then fully accessorized.  Only diference noticed was a slight point of impact shift due to the addition of mass to the weapon.

LtCol...

I have flip-floped on this topic over the last few years... I know (like  yourself) that *tests* can prove just about anything one wants them to -- I know about the test you mentioned earlier, wherein it was determined that the barrel/handguard assembly was flexing and causing broken bolts, and how those guns were ran to an extreme, or as you say it "plastic" state...

Anyway, here is what I know of such things... first I have to agree with one thing you said, that even bothering to free float the barrel in most cases is like putting lipstick on a pig... but as you may know, my interests start at 600 yards, so I was very interested in seeing what sort of flexing a FF handguard could do to a rifle -- We set up a pretty simple test that measured the error of alignment (angular) between the receiver CL and the barrel, with various loads and stress applied to a bare barrel, standard handguard and FF system.

I will see if I can find the results and post them, if not... it will be really easy to redo, very simple. It may be a week or so as the third sub is up a lot arond my house now, but I will get it.

I guess the way I look at this is that removing any flexing, even a tiny little amount can not be bad, but the MRP brought too many other problems to the game to make anything that it does right seem worth it, it's like dating a super model that is a democrat -- the Vltor system seems to address the constant rail and solid front end thing, while not introducing any new problems as far as one can tell... and still use all the regualr parts and goodies, if it does eliminate any questionable flexing, then heck, that is a good thing.

v/r
Gunz

I love a good debate!

Mr. blue and Gunz have two  perfect points, but I'm still favoring over to Mr. zilla's side.

I wonder where this debate will go?

Mr. blue, is KAC working on a mono design or is it something that has already been processed through the skunk works?

Vltor has a great design and  this is thread is getting better (pending debate).

Thanks!

lord

I  could state a bunch of negative experiences (some personal combat, albeit viet Nam era) re: the M16/M4 design, but one thing I know is true, is that the M4 with a RIS or RAS is a stout enough weapon not to be degraded by a little PEQ-2 and a flashlght.  I mean if it was that close to the failure margin, I think the 200,000+ RIS/RAS currently in use would provide such indications.

And yes, we did make some one-piece upper prototypes with rail forends.  However, we decided not to venture further for several reasons.
Firstly, Karl Lewis/LMT came up with a patent on the concept.
Secondly (as it has been observed above) such a design has considerable "scrap rate" potential.
Thirdly, maintaining interoperability with as many GI Supply System parts as possible.
And a few more...

And as far as testing is concerned, I can break a standard M4 bolt in a carbine/suppressor combo in less than 900 rounds with absolutely nothing on the barrel--no handguards, no rails/..nothing but cyclic rate boost on a part never designed for such stress.
If we are going to "error here," lets do it in favor of the Troops and provide them better designed (and material) bolts.
Nough said...

It's a wonder we are not all crunching over mounds of broken off bolt lugs as we trudge out to the firing line...

Just a bit off the subject but how much has any of the improved bolt designs helped with breakage issue and carbine gas systems?  Does anyone have any links to reasearch information giving us some stats on this issue?

Like Bigbore, I also think the mid-length gas system may fix the bolt breakage issues but I'd like to see some hard data to support this theory.

Actually, I'd like to see some stats on tests done on this issue to see how the mid-length, monolithic the improved bolts do on reducing these breakages types of breakages.

""testing" too many other "combined" variables to blame subsequent failure on a rail system squeezed betweeen the forend cap and the barrel nut.
What M4 Carbines and the 10.5's need are a higher cyclic rate spec bolt, and ammo tailored to burn efficienctly in the shorter barrels.
Nough said..."

I always thought the source of the problem was pulling on the vertical foregrip of a M4 carbine with non floating rail system under full auto or large amounts of semi-automatic fire like 10k rounds.  

If the bolt is really the issue, where are all the high ROF speced bolts availible?  

As far as floating a issue barrel; I would have to dissagree, as variable forend pressure cannot be discounted as a contributor to innaccuracy.  The floated barrel eliminates variables, and with M855 and a floated heavy chrome lined barrel 1/2 MOA is attainable.  With the Std M4 barrel floated, groups might be 1.25MOA-1.5MOA, but at least they aren't shifting POI 4-8inches with errant forend pressure.

Quoted: No carbine/rifle POI will shift 4-8 inches from pressure applied to a VG.

So what is the typical shift?

What a great discussion.  Let's keep it going!

i am very interested in this upper... holy cow...

i haven't read through the whole thread yet but I willl...

I wonder how they got around the patent of LMT... also, when you take on and off the barrel is there any POI shift?  I think the MRP was less than 1 moa, no?

Thanks!

Quoted: [What about a RIS or RAS with a a vertical grip being pulled on as hard as the shooter can pull to control the follow up shots? I've been reading posts about broken bolts, and most every rifle pictured has VG, and a CAR gas system. The last carbine class I was in, the only broken bolt was on a 16" CAR w/VG.   Has anyone here with a MID gas system broken a bolt? Last week I spoke with a local Dept who has been buying more and more replacement bolts from me.  I asked if they were shooting suppressed SBRs, and was told no, they just bought 6920s, and installed railed forends with vert grips. Coincidence?..."

Pulling that hard on a VG is bad news for accuracy period, as the barrel is not vibrating consistently due to varing ammounts pressure against it from round to round.  Pulling that hard is counter to basic marksmanship fundamentals period.
Not being an advocate of full-auto in the first place, well aimed rapid shots are my prefeence.  All I do with my grasp on a forend or VG is support & point the weapon.
It is hard for me to imagine pulling that hard with my support hand while trying to properly squeeze the trigger and maintain sight alignment.  And it is doubly hard to imagine that one could or would pull back SO HARD that you would be flexing the point at the upper receiver where the rear of the barrel extension terminates by the degree necessary for the extension lugs to missalign from those of a rapidly closing or opening bolt lug, thus causing an impact (lug against lug) that would break off a lug.  (I also think the the apparent absence of any reported barrel extension lug failures supports this opinion).  I mean if 100 bolt lugs have been sheared off this year, then there would have been an equal number of barrel extension lugs fail as well...Right?
Also consider the clearance and tolerance "gap" designed into a proper bolt lug to extension slot interface.  I mean this is not a tight fit (bolt lug to extension slot) like there is between the bolt diameter and the ID of the bolt carier.  There is a lot of space allowed there.  So the amount of deflection through leverage (considering the absence of any firm focrum point to focus any bending)  to not just close up these gaps, but totally missalign them to the point of a metal-to-metal impact, is just not going to happen if the buttstock is in your shoulder.
Sorry if this is off topic, but I did not want anyone to think I was hiding from opposing opinion.  I am jus vey busy with work.

The barrel extension has a tight fit inside the upper receiver. Pulling down on the barrel will cause the barrel to bend forward of the extension, it shouldn't be possible to misalign the barrel extension with the rest of the upper receiver. To do that, one would have to apply such a force on the barrel that it would cause the barrel extension housing in the upper receiver to malform. Come on....

Lets apply some math here.
If we imagine pulling down at the end of a 20" barrel, the largest stress would be directed downwards at the lower front edge of the barrel extension housing in the upper receiver. There is another force, but slightly smaller, acting upwards on the rear end of the barrel extension housing. Assuming the barrel extension is about 1.5" long, there is a leverage ratio of 20"/1.5" that multiplies the force acting on the barrel. If we hang a 20lb weight on the barrel, there would be 267lbs acting upwards on the rear portion of the barrel extension housing and a combined force of 287lbs acting down on the front of the housing, which is a cylindrical surface. The 0.2% yield strength of 7075-T6 is about 40000psi, so if we can agree that the contact surface of the barrel extension is more than 0.0072 square inches, then we can rest assured that the 20lb is not enough to cause yielding in the aluminium.

I could buy the rate of fire theory, but the bending theory looks like BS to me.

Excellent thread, once again.

Off topic question to ColdBlue, would it be possible to see pics of your E3 bolt?

thanks,
MN

Quoted: Excellent thread, once again. Off topic question to ColdBlue, would it be possible to see pics of your E3 bolt? thanks, MN

Onour web site (knightarmco.com), click on PRODUCTS and find it there.

Quoted: The barrel extension has a tight fit inside the upper receiver. Pulling down on the barrel will cause the barrel to bend forward of the extension, it shouldn't be possible to misalign the barrel extension with the rest of the upper receiver. To do that, one would have to apply such a force on the barrel that it would cause the barrel extension housing in the upper receiver to malform. Come on.... Lets apply some math here. If we imagine pulling down at the end of a 20" barrel, the largest stress would be directed downwards at the lower front edge of the barrel extension housing in the upper receiver. There is another force, but slightly smaller, acting upwards on the rear end of the barrel extension housing. Assuming the barrel extension is about 1.5" long, there is a leverage ratio of 20"/1.5" that multiplies the force acting on the barrel. If we hang a 20lb weight on the barrel, there would be 267lbs acting upwards on the rear portion of the barrel extension housing and a combined force of 287lbs acting down on the front of the housing, which is a cylindrical surface. The 0.2% yield strength of 7075-T6 is about 40000psi, so if we can agree that the contact surface of the barrel extension is more than 0.0072 square inches, then we can rest assured that the 20lb is not enough to cause yielding in the aluminium. I could buy the rate of fire theory, but the bending theory looks like BS to me.

This is a great discussion! As I said, I have flip-flopped on this debate in the past, but I base my current ideas on the testing done by the military into this very topic -- I am sure this is the same test that coldblue spoke of, it was in fact a "torture" test... and I would agree that the results could be suspect.

Just a couple of thoughts though... reading your post. A trivial thing, but I suspect that you are talking 6061 based on the yield figures that you gave -- naturally this is fine, and the proof strength (0.02 yield) of that alloy is about 40K, but keep in mind that yield point marks the transition from elastic to plastic state... the stifness of aluminum is pretty low and this may be more what we are talking about.

Also worth reading would be the old NACA Tech Note 879 for measuring the torsional strength of aluminum tubing, as the upper reciver is for all intents and purposes a tube -- it is noted that the plastic buckling failure should be about 60% of the yield strength (iirc, I don't happen to carry it with me) and that the elastic properties are far lower -- we are not talking about deformation or yield of the part, rather its elastic flexing.

A couple of other things to think about here... these figure need to take into account the normal operating tempurature of the part and something that I just thought of sitting here thinking about the relation of the part to the shape of a tube, naturally it is compromised by the ejection port... but one thing that I have yet to hear anyone suggest (and the event timing is right) is that the removal of the carry handle could have noticably degraded the structural integrity of the receiver? That "bridge" may have played a big part in the overall stiffness of the part?

I am busy training people myself... my day AND night job, but I will sneek off into the secret lab and do some table top tests -- I have thought up a jig to hold the parts and make some real measurments of whatever flexing there may be. Keep you posted...

Gunz

Quoted: This is a great discussion... Also worth reading would be the old NACA Tech Note 879 for measuring the torsional strength of aluminum tubing, as the upper reciver is for all intents and purposes a tube -- it is noted that the plastic buckling failure should be about 60% of the yield strength (iirc, I don't happen to carry it with me) and that the elastic properties are far lower -- we are not talking about deformation or yield of the part, rather its elastic flexing.... Gunz

I wonder if since mil-spec uppers are machined from forgings, if tubing parameters apply?  Especially when the end of the tubing is wrapped pretty tightly by a substantial steel nut.

And that gives me a another though...if one had to over torque a barrel nut to allow gas tube passage, might that "compress" the barrel extension and make the fit to the bolt lugs interfer if it was really heated up by exceeding the recommended rate of fire?

Tell us more Mr. Science!