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.
Bookmarks