You stirred up my curiosity.
This information is about pressure testing using a proof load cartridge. I think. So I'm not sure if this is what you're looking for-
"4.7.4 High-pressure resistance test. This test shall be performed during individual carbine testing in accordance with TABLE III, using a fixture for holding the bolt and barrel assembly per drawing 11837944."
That is part way down the page from-
http://www.m4carbine.net/printthread.php?t=50559

I couldn't find the "drawing 11837944" at any free webb sites. It may be available at some pay sites or at a government site. If you do a search for that drawing number you get almost only hits for the test manual that is referred to at the above webb site.

I kinda remember see a youtube video of a the military firing a M16 full auto until it melted down. I couldn't find it just now and I doubt that would be the fixture you're looking for.

I hope you find what you need.