I was just using "flow area" as a term for the "cross sectional area of the bore", i.e. the area which the gases are "flowing" through.

As an engineer, I can't ever help but consider everything around me the same way I would consider something at work, so when I think of throat erosion in a firing rifle, I tend to think of it in terms of transient combustion venting through a restricted tube, rather than simply a rifle firing.

I definitely agree that the .204 Ruger is a fantastic round, and if I didn't already own too many .223 rems and a few .22mags, it would certainly find a place in my safe. It's really the only popular commercial cartridge in its class (especially considering 32grn bullets), and has really came to dominate it's niche as the "lightweight" centerfire.

It seems to bridge the gap between the 22 cal rimfires and 22cal centerfires incredibly well, offering pelt damage more comparable to the rimfires, with an effective range nearly equal to that of the heavier 22 centerfires... Definitely an "efficiency round". Much like a .243win for deer, definitely enough power, but not as much "over kill" as the 30cal's, and a money saver on powder.