Upper Receiver Flex Testing - Part One
This is the test that I originally posted on another forum, but was asked to remove. I had to split it up in two sections as the forum only allows ten pictures per post. More tests, comments and some follow up stuff are in the second part of the post... wherever it is.
THIS IS NOT A TEST OF HANDGUARDS!
This is a test simply to determine if the idea of upper
receiver "flexing" is viable and to what extent different
types of handguard designs may attribute to that flexing.
Nearly all of the freefloat handguards on the market attach
in essentially the same way, to the barrel nut. This test was
conducted to see if that method did in fact cause the upper
receiver to bend or flex as some rumor it does.
This is not a statement for or against any product, just facts.
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Does It Flex?
http://img219.imageshack.us/img219/3...17small2mv.jpg
How I tested - This was about a dirt simple test, clamp the upper to a table, hang weights from the handguard and measure the flexing. I did some tests with loading weight on the barrel only, but he results are pretty predictable after seeing the handguard tests... and the handguard loading really is the big question.
Some "gauges" were made to show the alignment of the barrel chamber to the bore of the upper receiver -- these gauges are simple drill rod ran through the appropriate cartridge case; the end of the rod is slight under the bore size for the cartridge and taped to protect the barrel, it is forced into the throat area and the cartridge case (which fits snuggly on the rod) is shoved into the chamber, this is very solid and shows the position of the chamber very well.
http://img367.imageshack.us/img367/5...02small0bw.jpg
These gauges stick out of the back of the receiver -- when the weight is added to the handguard, the amount of deflection on the gauge is measured.
http://img498.imageshack.us/img498/9...37small5vu.jpg
This measurment is taken about 7.5" from the front of the receiver... assuming the barrel nut area is the fulcrum (just a standard), another measurement is taken 7.5" forward of the barrel nut to record the amount of travel in the handguard -- this was picked largely because I had to start somewhere and it is roughly equal to the chamber gauge measurment in distance from the chamber.
http://img525.imageshack.us/img525/8...39small0oz.jpg
Tests were done with five, ten and twenty pounds of weight... I realize that twenty pounds is a bit much, but it seems that in certain situations, it is not unreasonable to believe that a man could exert more force than that on the front of the rifle -- consider a twenty year old Marine, in his first firefight and add a GripPod or even longer VFG and while extreme, I don't think it impossible at all...
The suspension point for the weigth is at the same point as the forward measument.
http://img499.imageshack.us/img499/8...46small3qp.jpg
With the weight in place, I recorded the amount that the handguard was flexed down... and the amount that the gauge was deflected up -- on each set, the weight was allowed to "soak" for a few moments and then the load was taken off of the receiver to see if it would return to zero... this was repeated three times and in every case the measurements under load were the same and the gauges returned to zero when the load was removed.
http://img459.imageshack.us/img459/2...47small6pr.jpg
http://img455.imageshack.us/img455/5...49small8iy.jpg
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Test One - Because I had a theory going into this, I wanted to de a test where the front of the receiver was isolated and (as much as possible) the rest of the receiver was braced against flexing. For this set-up, I put together a CMT upper, 6.8SPC Barrel and midlength gas system with a DD 12 hand guard -- this was clamped to the table, secure from any flexing at any point aft of the front pin hole.
Supported Upper Receiver
http://img399.imageshack.us/img399/8...32small2hx.jpg
Findings - There was some flex apparent in the front of the receiver, but not that much really -- not enough to worry about or to cause the problems attributed to the flexing... this however, was good news, as it supported my theory about receiver flexing.
http://img525.imageshack.us/img525/7264/testone9iw.jpg
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My Theory -- I was actually glad to see that there was not a lot of flex in the front of the receiver alone... I have been looking at this issue and had the idea that the flexing was a real concern, but that it is the entire upper receiver that is flexing.
The upper receiver is held only at two points... the takedown pins. I suspected that when the handguard is forced down (as in the classic pulling on a VFG), it acts as a lever and is actually bowing the upper receiver in an upward sort of arch. So it is not so much the face of the receiver flexing, as it is the whole front part of the receiver pivoting around the front pin hole... the is exacerbated by the ejection port weakening the starboard side of the receiver.
http://img222.imageshack.us/img222/1179/arch4tv.jpg
Re: Upper Receiver Flex Testing - Part One
Pretty informative. I always wondered hire weight and tension played with the upper. Thanks for sharing.
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