Andrew Tuohy of VuurwapenBlog.com fame put together a fantastic article about the 5.56 and .223 cartridges. Unlike most of us, though, he actually did pressure testing using differently chambered rifles and a variety of ammunition.
There has long been a debate on whether it is safe to fire 5.56 NATO rounds in a rifle chambered for the SAAMI-spec’d .223 Remington cartridge. Thanks to the folks at LuckyGunner, where the article is published, Andrew was able to use actual pressure monitoring equipment to give a more than theoretical answer.
I highly recommend checking it out.