Upper Receiver Flex Testing - Part Two

[POF-USA]

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.
Best regards,

Frank
POF-USA

[K.L. Davis]

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,

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.

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.

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.

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.

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.

[twl]

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.

[Heavy Metal]

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.

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

[K.L. Davis]

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.

No problem... I hope my pseudotechobabble (tm) 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.

[UPSguy]

any new updates to this testing?

[K.L. Davis]

any new updates to this testing?

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.

[Ned Christiansen]

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.

[K.L. Davis]

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

KLD

[jmart]

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

KLD

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?