I ran into this at christmas and wound up learning lots I didn't think I needed. First, don't believe for a moment that 5.56 brass and 223 (bolt action gun) give you the same brass back. The difference is that the 223 Remington is a Sammi spec'd and adopted cartridge so there is a published standard. The 5.56 is not. Where the difference is, is at the headspace measurement. The datum line on the 223 is 1.466 inches from the case head to a place on the shoulder measuring .360 in diameter. Also the neck and shoulder junction is at 1.5573 from the case head to that point.
Many 5.56 chambers are longer than the 1.5573, some as much as .040 longer. <<<I learned that the hard way when I couldn't get my bolt to close on my 223.
Someone mentioned setting the die tighter to overcam and that will bump the shoulder back. This works, maybe, depending on the die set. It doesn't work on my Redding match dies. God knows I've tried. A local guy here told me to get a full length die and have the bottom turned about .025 shorter (basically grinding the die itself shorter) so the case can go farther into the die. Apparently this is a trick some people use on Thompson Center breakover guns.
Where headspace distance gets into a concern is when you buy once fired brass. I have read that the 5.56 headspace spec is 1.592 which is .037 longer than the .223 spec. I don't believe that number is true but some ammo suppliers are using that number for a spec, I don't understand why. Case gauges are nice but they don't replace a good set of calipers.
My first post here and I write a book, sorry
