[Unclenick]

First, I'll confirm what Std7mag said. I've deburred lots and lots of Lake City cases that were once-fired and still had their burrs well-preserved. I can imagine how, if the case had a compressed load, that pressure building in the center might cause powder grains adjacent to the burr petals to press back against them momentarily. I just haven't noted it for myself. I'd probably have to use a bore scope to do a before and after.

In my experience, whether or not you see an effect depends greatly on what powder you are using. Using Remington cases in my M1A, I found it would shoot 168-grain SMK's into about 0.75 moa with all the stick powders I tried in it. When I tried Accurate 2520, though, a load that had the poorest loading density of those I tried, the groups opened to about 1.25 moa. When I deburred the flash holes, they went down to about .75 moa like the other powders. Back then (early 90's) I was using Federal 210M primers and was unaware of the value of CCI magnum primer reformulation with spherical powders, so I don't know if I'd have got the same result using them or not. I can say for sure, though, that it made an astonishing difference with standard primers at the rear.

When I read Litz's observation about chamfering it reminded me mine were chamfered as the nose of a center drill was used to make the tool I have. It sounds like a pilot in an end-cutting end mill would be better.

One of the problems with burrs I ran into was that my Dillon Super Swage 600's anvil would tend to flatten them down (good), but sometimes bend them over the flash hole (bad). If you use a swaging tool with an anvil to remove military crimps or to widen tight CIP spec flash holes, you want to do any flash hole reaming or drilling afterward. Makes me wonder if the best way to remove burrs might just be tapping the end of one of the Lee hand decapping rods against the bottom rather than cutting at all. Something else to try.