The MUR was tested and proved to be stiffer than normal flat top uppers and that was when it was made of 6061. Now, the MUR is made of 7075 which will make it even stiffer. Unfortunately, the threads where K.L. Davis tested upper stiffness has been stripped of its photos but here is a link to a thread that has links to the tests and a couple of other pertinent discussions.
https://www.m4carbine.net/showthread...per-Flex-tests
Short version is, the MUR is quite stiff and doesn't weigh much more than a standard forged flat top. I have one on a precision build using a Lothar-Walther 20" HBar barrel and am very satisfied with its performance. The MUR also allows the choice of any free float handguard, which is a must for me because I don't like aluminum for handguards. I don't know anything about the M4E1 upper but I can give my recommendation to the MUR as a precision upper, especially if a suppressor will be used
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