Be aware, though, that the flash hole is typically punched. It may not be perfectly centered and often has burrs on the inside. For the best concentricity, you may still wish to drill from the outside, and start the hole with a center bit.
Regards,
Eric
I was under the impression for some odd reason that the web of the average rifle casing was much more conical than flat, and that tended to guide the bit to the center. In the spirit of "a gram of finding out is worth a stone of talking about" I took a unfired USGI 30 cal shell, pulled the bullet, poured the powder down the sink and followed it with a dose of water, wrapped the casing in a paper towel, clamped in a vice with the open end pointing away from anything (including me) put a round tip punch on the primer and tapped it. Moderately loud bang ensued, but as the gas had plenty of room to expand and nothing to push, no other results were observed (or anticipated). I then took said casing over to Mr. Disk sander and removed half of it lengthwise.
A quick scan proved that Eric has a better recall of casing construction than I do. I have however to date not had any holes drilled from the inside of the casing that were unsatisfactory to me in their concentricity. This may however simply be due to lower standards on my part.
Just in case anybody is still reading this, a few facts about modern (post 1950) cartridge primers. The first non mercuric, non corrosive primers used Lead styphnate (lead 2,4,6-trinitroresorcinate, C6HN3O8Pb) as their primary explosive. This compound is a primary explosive, meaning that the chemical reaction is propagated by the shock wave traveling through the compound at about 17,000 fps. This is opposed to the deflagration of gunpowder, where the flame front progressing along the length of the charge propagates the reaction, producing a much lower pressure gradient.
Recently, Environmental pressure and levels of lead contamination in firing areas has pushed development of other lead free compounds, but all share the same basic effectivity.
Bottom line, primers produce an very high pressure spike when initiated IF THE GAS IS CONFINED. Given an unobstructed path to expand, the relatively small volume of gas produced is relatively harmless. HOWEVER... if something (like a jewelers screwdriver or punch) is significantly restricting the egress of gas, it will be almost instantly accelerated to very high speed. At short distances, they can be quite lethal.
"insert your favorite legal disclaimer here"