Upper Receiver Flex Testing - Part Two

AR/M4 AR Technical Discussion
21

K.L.Davis test are to determine flex in the receiver.
His results have shown some interesting results.
The amount of flex on a receiver with hanging 20 lbs on the rail system,
then adding heat showed flat top receiver flex / movement as much as .016".
Adding heat to the testing helps to test as if the weapon system is operating.
It will be interesting to see the results with the use of a vertical grip.

His test results show that a carry handle upper has less flex / movement of the receiver when hanging weight. The carry handle upper only flexed / moved .0015". I beleive this is because the bridge support of the carrying handle. I hope his test results will help to prove why.

What’s another interesting point of his test in his very rigid looking fixture. His fixture looks to be stronger then any lower receiver I could think of.
I would have to assume his fixture would help to reduce flex / movement of the upper receiver during his testing, compared to using a standard lower receiver.
Keep up the great work K.L. Davis. Your test result could help to improve product designs. :slight_smile:
Best regards,

Frank
POF-USA

22

Yep, the measurements are taken about 7" back… compensating for this is simple math however, it could be any distance from the chamber and all that needs to be done are the numbers adjusted to consider that distance – of course at some point, there would be enough flex in the “gauge” to become a concern, but it was not with this setup. The offset of the extreme edge of the bolt face is actually less than 1/7 of the gauge reading… iirc it was a factor of 0.06 of the gauge reading.

Keep in mind also that this measurement is stacked… you will see that there should be an equal movement in the opposite direction, 180 degrees from the extreme movement in one direction, but obviously there can not be.

If you think of it as a teeter-totter, it is like this… say that the gauge shows there should be a 0.001 inch movement (in relation to the bolt face and the barrel extension) and this movement is constructed to be in the direction of the muzzle – naturally, there should be a 0.001 inch movement at a point directly opposite of this, towards the rear of the rifle – but, in this case, on end of the teeter-totter is resting on the ground and can not move, so you force the fulcrum point to move forward the distance of the gauge measurement and the acutualy measured movement of the bolt face (agian, in relation to the barrel extension) is in fact 0.002, making up for the equal and opposite movement that could not occur.

Not really sure how the gas rings would have anything to do with this… the bolt bearing surfaces are forward and aft of the gas rings and there should be no lateral load on them at all – if everything is made right anyway.

Also, as I am sure you know, there is not any real “play” in the bolt… sure, setting on the bench there is some slop around the bolt in the receiver bore, but the rifle is designed to use the top cartridge in the magazine as a bearing surface – this in fact forces the bolt “up” into the top of the receiver bore and there is no slop or play… granted, the spring pressure and mass of the bolt assy could be overcome, but keeping in mind that work is a product of force and time, I do not think there is emough of the later to really give this a lot of concern.

The rule of thumb is that Al expands at a ratio six times that of tool steel… but given the design of the barrel mount and the fact that the barrel nut (should be) is steel, this is a non concern.

Under heat, there have been measurements at the gauge that would reflect a combined “gap” of 0.003 at one side of the bolt locking lugs – does this mean it is a cause of failure? Not sure, but it is enough to at least give consideration.

The purpose of the test (originally) was to examine the theory that the barrel does or does not flex in the receiver under load/heat – you know there was a lot of speculation and jabber about the idea (both for and against), so I just wanted to sit down and present some sort of repeatable test that shows if the idea is in fact sound, or garbage…

Given that more than one manufacturer is working to strengthen the missing “bridge” of the flat top uppers, it seems that the presentation was of at least some value.

23

Interesting.

I really appreciate your detailed response, and your addressing my points individually.
That was really nice.
I’m glad that my comments didn’t offend, because I intended them to be as helpful as possible, from the way I was reading the procedure.

Always interested in seeing what improvements can be made, and how they will affect things.

24

Right Forrest, you are introducing tensile forces on the upper with a bipod and compressive forces with stress on the handguard.

25

No problem… I hope my pseudotechobabble ™ was understandable?

BTW: no offense taken at all… in fact, most can rest in the comfort that I am perhaps too dull to realize when someone is trying to offend me.

26

any new updates to this testing?

27

Not as of yet… sorry. I have been so two-blocked lately that I have had little opportunity to finish the couple of other posts that I have working on here – trust me though, I am working on things still.

Between Truth, Justice, The American Way, influencing young minds and damaging young bodies, getting ASOSA back on line, a dog that has had more surgeries than Cher and wordsmithing my retirement letter!!!

…some things get back shelved for the time being.

I think the jist of the testing is there though, flat top receivers do flex. There is some more info that I will try to get posted, but I set out only to answer once and for all an argument that was going on in the development field. I think it was answered.

I am sure that I can get a MUR to test… Hopefully this will give a good idea of how the “beefier” uppers hold up, but the bottom line is that A2 and Monolithic uppers are the champs as far as strength goes.

28

Blimey!

I just don’t quite know what to say here.

KL, your testing is over the top. Just the kind of thing I love to see, but to be honest, although I’m sure it happens in the firearms industry, it is not often shared with Joe AR-head as you have done… thank you for your work and for sharing it.

29

Over the last few days, I have had several folks ask if there was going to be more receiver testing done… so, hell, why not?

I have started to ask around for new stuff to test – so far I have shaken the trees to see if I can get a MUR and production VIS… as well as the new billet upper from LaRue Tactical – so it looks like this is shaping up to be a test of the “improved” designs that are out there.

Great stuff, and I am looking forward to seeing what improvements have been made… I am sure that everyone will be pleased with the efforts from the industry and hope that more good things are still upstream.

If anyone can think of another entry that should be tested… feel free to add on your comments and ideas. I would like to round up a billet upper from Oberland Arms and a couple others come to mind – but let’s see what the forum is thinking!

UPDATE: Well, an Oberland Arms upper is in the test… thanks to a generous donor :cool:

KLD

30

I’ve followed this topic since the beginning and find it pretty fascinating. Along the course of your testing, have you been able to correlate the amount of flexing with either reliability or accuracy degradation?

31

< shouting Brit accent >
I’d like to see…
< / shouting Brit accent >

A MUR w/DD Lite rail next to a the LaRue receiver w/LaRue rail.

I wonder if the mounting system of the DD makes any difference.

I wish there was a way to apply any of this testing to actual effects on functional firing weapon; what difference does any of it really make?

With my yellow glasses - I see folks who must be putting much more than 20lbs on the end of a fifle length float tube. Standard receivers, loads loaded as hot as possible, and I cant think of a problem ever being had.

I would have to imagine a yellow glass shooter slung up prone, is puttin way more stress on a tube than any soldier with a carbine, even under stress.

This testing is neat, but in what way does any of it apply to anything a simpleton like myself can relate to?

It would be cool if you could do your testing with hot uppers, or mount something like a laser level on the receiver and a aiming point on the front sight of a match rifle to see how much its flexing from a sling.

32

I asked myself that very same question when I started reading this thread, but the more I thought about it, it seemed the tension a sling would place would be more of a compression than a bend. The bulk of the force’s vector is coming back along the axis of the tube (down the sling and toward the shooter’s bicep), comparatively little is going downward.

I do share your same curiosity though about how much flex needs to occur before reliability and/or accuracy detriorates. Between this thread and the “migraine inducer” Larue billet receiver thread over at you know where, it would be neat to figure out between the two what the correlation is between flex/dimensional errors and accuracy/unreliability.

33

I’d like to see a high rise upper added to the test. DPMS has made them over the years but it looks like RRA is the only game now.

34

Okay… as of last night, I have an Oberland Arms receiver.

This morning I got word that a production VIS and MUR are on deck as well. I have not heard from LT… probably not back in the shop until monday.

I will hit up DPMS and RRA to see what they want to submit, and I have a couple of emails that may possibly bring in a couple of others. I will see if DD wants to send a new handguard to test the mounting method… but it would have to be ran against a “traditional” mounted free float on the same upper – running the test on two different manufacturers introduces a variable.

So… the reason I started this testing a couple of years ago was just to see if there was flexing in the upper receiver – the camps where split and you either thought there was, or thought there was not. I like to think that this testing put that to bed… and most manufacturers and end user organizations that I talk to now, subscribe to the results of the test.

I don’t have any direct correlation to the amount of flex v. mechanical problem… but I do know that some of the shops that have replicated or continued the tests claim that it is at least a factor in bolt breakage – anything that I can add to this is purely anecdotal. I am also told that when the test is taken to 400 degrees, the flex is very severe!

We all know from our days in basic training, that the traditional sling to the FSB can flex the barrel enough to move POI… but the test here was to see if loading the handguard caused the front of the receiver (and therefore the relation to chamber/bolt) to flex – interestingly, but easy to understand, is that the problem is manifest under “up and down” loads and not nearly as noticable under any sort of “side to side” load.

I would say this… cutting off the carry handle reduced the structural integrity of the upper, no doubt about it – does it matter though? This is just me thinking, but we have had a really bass ackwards approach to this for a long time – think about how much you would be willing to pay for a good bolt action receiver? The upper receiver is functionally the same thing, but if someome was selling $35.00 flat tops on “The EE”, they would fly off the shelf (if they had M4 cuts) – and in spite of that, we are willing to pay $200.00 for a lower. Hey, if the hammer pins holes are straight, the mag well in spec and the receiver extension lines up – that is all we can expect from the lower… I guess because it is the serial numbered part and carries “The Name”, we throw the extra cash at it?

Again, I am not going to replace every flat top receiver just because it may flex or whatever, but if I had a choice for a new build – that was going to be used in the defense of life… I would opt for the best upper I could get my hands on.

Which one is that? Hopefully, in a couple of weeks, you can make that decision based on what you read here.

35

I was talking about NM Service Rifles with the sling attached to the float tube. There are some big guys putting a LOT of pressure on those tubes - even though its not forced directly down, things are still bending. Same can be said for the match uppers with float tubes built an flat top uppers. Amount of flex as temps increase - these guys arent shooting more than 10 rounds at a time. Seeing scores shot with ARs - I dont for a minute believe the flexing alone has any noticable effect on accuracy.

think about how much you would be willing to pay for a good bolt action receiver? The upper receiver is functionally the same thing,

Apples and oranges of a comparison for this testing. It would be nice to compare the consistancy of the internal spec dimensions of receivers. A nice black (so it matches the lower) upper with M4 feedramps, for $35, that the barrel extension fits in like a hot dog in a garbage can, and bolt carrier that rotates around when closed is another thing…

But yes, they would sell because the folks buying them dont shoot more than 50rds a year off a bench at paper plates 50yds away;)

Again, I am not going to replace every flat top receiver just because it may flex or whatever, but if I had a choice for a new build – that was going to be used in the defense of life… I would opt for the best upper I could get my hands on.

Is the best the enemy of good enough? What will you use to determine what is “the best”? How many of each can be tested to find who is more consistant?

I would hope this will break down as best, good enough, and decoration.

Best - I would determine by strength, and consistancy of “spec”.
I would expect the “billet” pieces to come out as the best.

Good Enough - I would think Colt, and CMT - because of consistancy to specs.

Decoration - the gunshow specials where all the pieces fit with “room to spare”.

36

Best = Value

Value = Need/Cost

Need, and Cost can be rated 1 - 10, 10 being highest.

37

After seeing newer and “better” gear in five years of classes and staying current with the boards, I came to much the same conclusion; " Is the best the enemy of good enough?"

The kind of testing that would be required to determine the “best” would add significantly to the cost of these parts. (Imagine two identical valves, one certified for use in a nuclear power primary system and another for a drain line on a surface ship; the former costs 2 to 5 times the latter, perhaps more.)

I cannot afford perfection in this arms race. Colt lowers, even the lowly Sporter I’s, with uppers made from Colt barrels, Colt upper receivers, and Colt bolt carrier groups “work pretty good”. At this point, money spent on ammunition for training and practice is arguably better used.

38

Agreed, Sub. If I find myself going cyclic on the bigger, better, faster loop I go back to Kamm’s law of diminishing returns. The difference between having an AR that runs and not having one at all is much larger than the little hoo-haws we deliberate on for waaaay too long.

That said, it’s enlightening to see technical comparisons being done even if it’s essentially esoteric.

39

To clarify… I hope to find the design variation that provides the most strength – as for which is “best” that is ultimately up to the end user.

Steve, you know that you and I are both very pragmatic when it comes to this stuff… hell, I don’t have nearly half of the high zoot stuff that is out there on my personal guns. But we have both seen the strange alignment of priorities that a lot of the end users subscribe to – granted, a lot of them only shoot 50 rounds a year and the closest they will ever get to their gun being in “battle”, is if it falls over in the closet and scratches their wife’s favorite high heels.

But, it used to be barrels… remember how folks would say “I want this name brand lower; with a custom plated NM bolt; a 200 dollar stock and another 200 bucks worth of extra grips; three lights; 4.7 yards of rail on a 300 dollar fore end; at least three sighting options, one of which is optic and cost more than the rifle itself; I’ll need a 100 dollar flash suppresor; a sling that comes with a video of how to use it… oh, and what is the cheapest barrel you have?”

Now, in some sort of knee-jerk reaction I guess, one can not show up on the range without a gun that has the latest ubertube…

That said, I am not sure that any of the crop of new uppers are any better… yet.

For those that don’t know me… When I first really started into the gun thing, IPSC was a budding sport and the number of custom 1911 builders could be counted on one hand… the “martial artist” and the “roonies” were just starting to go their seperate ways and for most guys, extended safeties, beaver tails, better sights and a trigger job was about as exotic as you could get – there was, however, a group that was putting weights on the end of the barrel… they even ported the weights and played around with cartridges once thought extinct, this small group of guys mounted over sized goodies on their guns and welded/filed/milled in different shapes… it really was an interesting time.

I worked part time as a gun plumber for a small shop in Phoenix then, the Hassayampa River Shooters and the Cactus Match League would meet in the evenings and we would have a great time… a few would sit on the picnic tables and preach the ways of our particular following, and young shooters would sit wide eyed and soak it in. I was doing pretty good with “custom” pistol building at the time… well beyond just sights and trigger work, I had ventured into welding up and fitting barrels, extractor tuning, lug cutting and ramping and releaving – truley state of the art 'smithing.

Then, one night as I was sitting on the tailgate and discussing the virtues of my wares… a kid that had shot earlier that night (and not very well mind you) walked over and said “I have $200.00 to spend”… he held out his mostly stock AO 45 and asked “How do you recommend I spend it?” – well. I answered honestly and pointed him to a guy that worked for Phoenix Arms and said “Go buy 200 bucks worth of his reloads and practice more”… you know that Charlie Brown whaa, whaa, whaa sound? I swear you could actually hear that after I told him this.

The next week, I did not have nearly the following of new shooters hanging around seeking wisdom… it seems that my suggestion, while it no doubt would have improved his shooting, did little to improve his appearance on the line… I learned a lot that night, but I have not changed. If something truley offers advantages that offset the price, I will wholeheartedly recomend that item… if, however, something is “fluff” or simply bragging rights, you won’t find it in my favor. Therefore, I do these sort of tests so that I can make sound recommendations based on what I know to be facts.

In honor of a good friend recently lost… the first Rule of Gunfighting is: Have a gun.

That simple axiom speaks volumes.

40

Update…

The test should go down soon, I have two short deployments this month, but can probably find time between them to get it all taken care of.

I have commitments for product from:

Daniel Defense
Vltor (New VIS and MUR)
Oberland Arms (private owner)

and I am pretty sure that RRA and DPMS won’t shy away.

The testing will include how much strain can be applied via a VFG. Should be lots of fun!