Upper Receiver Flex Testing - Part Two

AR/M4 AR Technical Discussion
1

Here is the rest… work continues on this, see the comments at the bottom.

Test Two – This was a duplication of the first test, this time with the upper receiver supported by “pinning” into a fixture that represents a lower receiver.

Conventional (non-supported) Receiver

Findings - Here we see an increase in all of the measurments, with a noticable increase in the deflection of the chamber gauge – further indication that the flexing of the front of the receiver is valid and demonstrable… but are the measurements enough to support a real concern?

Test Three – Leaving everything in place and under load from Test Two, I used a small torch to apply heat the barrel nut/chamber area of the upper receiver assembly. Lightly feathering only the chamber area with the torch, I took the temperature to 200 - 220 deg (F) – not at all an unrealistic temperture for the rifle. The results here were very interesting, the dial gauges reacted immediately to the heat. As this was still an elastic state for the metal, it was interesting to watch as the upper cooled… the measurements returned to the original readings of Test One as the receiver returned to room temperature.

This test (I believe) induced enough alignment error to believe that not only the reliability, but the structural integrity of the weapon may have been compromised.

More Theory – I had fully expected to see the results that I had seen up to now… I suspect that the defining moment of this problem was actually the removal of the carry handle. The time frame for the apparent increase in some of the issues attributed to the “receiver flexing” does seem to fall in line with the increase in flat top uppers being available. While perhaps not designed that way, the handle is a integral arch that ties the front of the receiver to the rear anchor point (take down pin) and we sill see, adds a LOT of rigitity to the upper receiver.

Test Four – An A2/Carry handle upper is built with the same parts as the flat top and pinned in the same fixture. I went straight to the twenty pound test with this…

Findings - The carry handle appears to stiffen the upper receiver a great deal, the deflection at the chamber cauge was actually just over one-onethousandths of an inch!

Test Five – The next test was with a “piggy back” type mounting handguard… for this test I used a CAS-V system from Vltor on a CMT/Stag M4 style upper – again, I went straight to 20 pounds on the test.

Findings - Although there was a lot of flex in the handguard, note the chamber gauge deflection was very close to the A2 Carry Handle upper at just slight less than 1.5 thousandths of an inch!

Test Six – Of course the test would not be complete without a look at the new monolithic upper receivers… I only had one such part in my hands during the test, but decided to see how it does.

Findings – I am going to hold off of specific measurements right now, as the unit I have is a prototype and I am told the production piece will be slighty different… I also hope to compare other entries in the monolitic catagory in a side-by-side match up.

To be fare to everyone though, I will say that both readings were better than those recorded under the twenty pound load on the fully supported receiver in Test One.

I can say that it is very rigid… the monlitic design appears to offer a far more rigid handguard section than is avaiable from any of the above combinations and the measurable receiver flexing is far less than what was noticed from the current flat top upper and barrel nut mounted hand guard systems.

update: the readings for the prototype VIS were handguard deflection of 0.011 and chamber gauge was 0.004

Test Seven – A test of a new issue KAC RIS system was interesting…

Findings - The RIS did very well really, this is mostly for comparison, but the handguard deflection was 0.0014 and the chamber gauge deflection was 0.007 – this is about as good as the fully supported receiver in Test One and better than the unsupported receiver in Test Two.

Conclusions – This is not the end of the testing… but offers some pretty telling evidence. As stated, I hope to do more side-by-side testing with the monlithics in the near future, as well as some range time with different designs (I have designed a jig that will intentionally stress the upper receivers and see if I can not get them to break!) – but for now, I feel comfortable with the folowing.

The upper receiver “flexing” issue seems to be real… I believe that the removal of the carry handle does make the platform subceptable to “bowing” the upper enough that it could effect the reliability or integrity of the weapon.

Granted, this will vary from rifle to rifle and is something that most likely would only be experiencd by a shooter that is a very stressful situation and has fired the gun to the point of being quite hot… but this seems like the absolute worst time for it to happen. This flexing also seems to only be an issue on guns that use a barrel nut mounted handguard system.

All of this is most likely a non-issue for 99.9% of the people reading this… The likelyhood that the combination of the weapon configuration, individual user and appropriate situation will all come together exists almost solely on the battlefield… and even there is would have to be the exception rather than the rule – but, if you prepare for the worst, it is not as much of a surprise when it happens.

Some other notes:

The strongest upper is the old A1/A2 carry handle upper – I did not test a removable carry handle or one piece optic mount as a possible “bridge” on purpose… as I felt that even if it did stiffen the upper receiver, it would only be as good as its current mounting and can not be relied on as a remedy.

The carry handle also seems to serve well as a heat sink… more on that in the future perhaps.

Yes, there is someone already working on a receiver that uses an itegral optics platform to strengthen the upper… as well as a few lesser known things already out there.

The “piggy back” style handguards do not stress the barrel nut, while it is debatable if they actually provide an additional rigidity to the top spine of the upper receiver, the example used in this testing did not seem to flex the upper receiver. These systems however, do seem to have as much or more rail flex than the current barrel nut mounting designs.

02MAR06 - No doubt that the missing carry handle weakens the receiver… some model work indicates that the “sidewalls” of the upper receiver provide little reinforcement – the starboard side is a washout with the ejection port taking a large piece of the structure away and it seems that the weak spot on the port side is just below the cut for the cam pin to clear. Most likely the sides are bowing out under load (in the area indicated in the pisture) and the only real support is the top section of the receiver, which allows the front section to flex as it pivots on the front takedown pin.

07JUN06 - Finally get to put this back where people can see it! Computer models confirm the idea that the receiver is bending just behind the front lug under load. Some idea for a reinforced receiver are being looked at. I will keep this updated as I get new stuff.

2

K.L. Davis,

Great information!

I hope to read more testing with heat from you.
Weapons heat up if shot hard and fast (Example: Beta-C dumps in full auto).
Your test seem to indicate that the hotter the weapon gets
the more the chamber indicator would show movement.

I wonder how much movement/flex of the chamber indicated reading you would have, when the weapon is hot and using a vertical grip mounted at the front of the rail system and pulling it toward the receiver?

Just a suggestion that you may want to try:
Maybe you can test this by vertical mounting your upper receiver and fixture,
then install the vertical grip at the muzzle end of the rail system, then hang weights off the vertical grip.
Use a torch and a infered heat sensor gun to record heat and indicate movement in the chamber. This type of a test would reflect how a weapon system using a vertical grip may induce movement/flex in the upper receiver when the weapon system is hot.

Keep up the great work here. Your information will help others. :smiley:
Best regards,

Frank
Patriot Ordnance Factory (POF-USA)

3

Mr. Davis excellent post. I look forward to reading more of it this weekend.

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Frank,
Instead of vertically mounting the upper, which changes the the setup, and any gravity effects (even if minimal). Why not use a pully? Tie a cord to the VFG run it over a pully then tie the weights to the cord.

This is the idea (excuse the crude graphics):

Mr Davis,
Thanks for posting these threads! I had been wondering how your experiments were going.

5

I have a real simple idea to measure the force that is applied from a VFG, I just have not had time to do anything with it yet. I am deployed for the rest of the month, but maybe I can get someone to do some tests and see what sort of force we are looking at. Thanks for the positive feedback!

6

Your drawing looks great, and I’m sure it will work. I would be nice to determine the amount of weight/force a operator puts on the vertical grip when pulling back during use, and match that weight/force for your test. This type of leverage may induce flex in the upper. Your test would show if this is a issue or not.

I would continue to test as you have been. Take measurements when cold for a base, then take measurements at different temperatures to see how much movement/flex the upper indicates with heat.
Best regards,

Frank
POF-USA

7

After thinking about this last night, I would love to see what pressure a bipod puts on FF system from a prone position. I’m ASSuming it’s along the same lines of a foregrip. But maybe it would be less as the rifle is in a rest position. What do you guys think?

8

I thought about this, but never gave it a lot of worry I guess… I may be guilty of personal habit making me not look into it much though. I am an advocate of the bipod being located pretty far back on the weapon, largely so you dont have to make all sorts of movement to reach out and fiddle with it. At times, I will set the left leg short and hold the bottom of it with my offhand, this is a great way to level the rifle quickly.

Anyway, that position probably puts little strain on the weapon, as the “pivot” is so close to the barrel nut, but this is a something to look into for sure.

Most will be surprised by the amount of force that can be put on the rifle from a VFG – keeping in mind that we are talking about an 18 year old guy that just finished MC basic and is a situation that, as we like to say, makes him water tight… pucker factor you know!

9

I never thought of that. Your right the pucker factor will really put some stress on the forearm.

I’m sure the bipod puts almost nothing on it compared to the above. But I like mine way out front. I’ve noticed that it does change the POI over the POA if I lean far into the stock. I’m a big guy at 260lbs. I use the harris so it likes to jump around/slide back a little.

10

The difference between adrenalin induced combat strength, and shooting on a bipod would make it apples and oranges. I don’t doubt that POA/POI shifts are consistant with what you are saying, but I don’t think it would be anywhere near the degree of stress on the same parts.

I can’t help but wonder if the answer to this is going to be a redesigned receiver with support ribs in additional areas. I think the problem in that regard becomes getting the military to make the change (less than likely). The more realistic answer may be in the upcoming requirements for military rails.

Are there any stats available to show number fo failures in use?

11

Interesting conclusions on the KAC RIS. So much for the “sucks completely” theory Ive seen thrown around lately.

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Stick I agree with you. The amount of stress that can be applied with a forearm is greater then a bipod.

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The force the bipod applies is going to be different. It’s going to more of an ‘up force’ rather than a back or a downforce.

While the bipod doesn’t push ‘up’. It is going to be static and the weight of the receiver is going to be pushing down on it. The end results is it will look like an ‘up’ force at the point the bipod is attached.

14

Thats correct Forest. I failed to mention the barrel is a .625 barrel. I tend to pull the stock in tight then lean into the gun. This applies upward force on the bipod/FF rail and tends to drop the barrel a slight amount. Currently I’m changing that barrel out for a stainless heavy 1/8. I hope this will resolve the problem completely.

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K.L. Davies,

A closer look of your report shows the Cav-V and the Carry Handle upper had the least amount of indicated movement of the chamber .0015.
Did you find why the Cav-V had less indicated chamber movement /receiver flex? It would be great to hear your findings on this.

Have you conducted a test with a vertical grip, and test with heat?
I think this is the test you will find the most movement, compared to the hanging weight. Having a vertical grip mounted by the muzzle will introduce alot of LEVERAGE, when the vertical grip is pulled back toward the receiver during use. I’ would think you will see the most indicated chamber movement / receiver flex, when conducting this test compared to the hanging weight test.

The test results showing the RIS rail moving .0014", with weight. I would assume this is due to the front on the rail is supported by the barrel and is not a free float rail system like the others tested. The indicated chamber reading / receiver flex showed a reading of .007.
Keep up the great work. I look forward to more updates. :slight_smile:
Best regards,

Frank

16

K.L. Davies,

Have you done any testing with the vertical pistol grip yet?
Looking forward to your test results.
Best regards,

Frank
POF-USA

17

Yes… I just got back from a short deployment and I am turning around and heading out again soon, but there is some new information that needs sorted through and posted.

The VFG puts much more stress on the upper receiver than a rifle that does not use one – the reason is that without the VFG, the stress created is limited to that the operator puts on it by “pulling” the barrel down (or off axis in some direction)… this is not really that much, as it is not a good shooting technique.

The VFG however, promotes grip strength and seating the rifle by pulling back on the grip – all of this force is transered to a leveraging force on the front of the rifle, due to the VFG acting as a lever, perpendicular to the axis of the rifle.

I will get some more stuff up as I get time, this next month is sorta busy for me, but I will get what I can…

18

Your doing an EXCELLENT job with the tests K.L.!

Some of the best AR PHYSICS education I have had in a long time…

Rmpl :smiley:

19

Thanks for the reply.
I agree with you that the VFG puts more stress (leverage) on the weapon system. Your testing will prove if this is true. I believe we will see more movement with the VFG and heat in regards to the receiver flex.

Come home safe, and keep up the great work.
Best regards,

Frank
Patriot Ordnance Factory

20

It appears to me that your chamber gauge is being measured about 7" behind the bolt lockup area.

The length of this rod adds a significant increase to the measured movement, due to the angular calculation making a larger measurement, the longer the rod is(further from the chamber).

As a result, the actual movement at the lockup area, is far less than the measurement being taken at the end of the rod behind the back of the upper receiver, because of the length of the rod, and its greater movement in the arc at the end, than at the actual area of concern.

I think that this needs to be taken into account, and measurements taken at the bolt lockup area, by inserting the dial-indicator thru the ejection port.
That is, if we are looking at this phenomenon as being a potential cause of misalignment during bolt unlock, possibly causing excessive and uneven bolt wear, or breakage.

The actual movement at the chamber or lockup area will be about 1/7th of what is being measured at the end of the rod, due to the angular increase in where the measurement is being taken. It is quite possible that this much smaller movement may easily be taken up in the play available at the gas rings, which allow some small “wiggle” movements of the bolt inside the carrier.

If we are looking to determine a possibility of carrier binding on the upper receiver after unlock(because of this flex), then we must measure the difference in the receiver carrier channel from the front of the ejection port, to the rear of the receiver, to determine a “binding point”, and determine if this is beyond the clearances allowed for movement of the carrier in the channel. If we have a “bind-point”, just sliding the carrier back and forth in the channel, should allow you to feel if it is actually a problem to consider.

My “gut feeling” here is that any of this flexing going on can be accomodated by the play in the gas rings, and in the clearances allowed around the carrier, in most of these cases seen here.
Perhaps it may cause some uneven wearing to a certain extent, if the gun is always fired with this kind of pressure on the ends of the handguards. However, I think that in general use with heavy pressure exerted only in some of the circumstances the weapon will see, it may/may-not even be a concern.

Regarding, this worsening when heated, it is known that aluminum has a different expansion rate then steel, and the aluminum receiver-hole holding the barrel extension will enlarge(loosen up) to some extent when hot, thus possibly(likely) causing the phenomenon of worsening this deflection measurement when hot.

Just my thoughts.