A hot barrel/chamber will if anything decrease your headspace. However you'll have to be firing a belt feed full auto long enough to get the barrel glowing and drooping to make a significant change. You're more likely to have sticking cases in your chamber effecting you first. That's a big reason the military 5.56 chamber is different from a .223. It's made slightly larger to allow soldiers to continue firing when the M16/M4A1 are hot. When you start having extractration problems, it's time to let your weapon cool down.