Back in June, there was an article about the G36
http://bearingarms.com/awkward-moment-hk-fanboys-find-germans-call-g36-rifle-doesnt-shoot-straight/
The rifle’s point of aim shifts when the barrel heats up under sustained use. Like in, you know, combat...
Not to worry. The Bundeswehr will come up with a Zeiss red dot sight with an infrared barrel temperature monitor and onboard aimpoint computing to correct for the drift.
No argument with your comment, but the article used the phrase “operate properly”, which I would equate to internal parts heating up to the point of creating an interference fit and jamming. Maybe both are happening.