The other thread was locked, so I started this one to continue the discussion.
I went and looked up the chamber lengths for 223 Rem and 5.56 NATO.
223 - 1.772" to 1.776" (case head to case mouth based on 5 different reamer makers)
5.56 NATO - 1.772" to 1.775" (case head to case mouth based on 3 different reamer makers)
The recommendation to intentionally use cases that are 1.780" long is potentially dangerous to others. The case is likely to be 5 to 10 mils too long for the chamber and are 30 mils longer than the recommended trim-to length.
Where these two chambers differ the most appears to be in the lead and freebore. However, these features are beyond the end of the chambered brass. Check the sketch from Colorado Osprey about half way down the page at this
LINK
What is key is to measure and know what your chamber's dimensions are. I've measured my chamber lengths (so I know when I have to trim my brass) but don't have the info with me.
