HK 416, MR 556 InRange TV Mud Test.

[Arctic1]

There is a standard for mud testing, in Test Operations Procedure (TOP) 3-2-045.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a481861.pdf

Section 4.5.5, page 39.

As someone who works with submitting weapons for gov solicitations where requirements include passing these tests, these You Tube tests are not anything but entertainment.