Using Nosler's numbers for 40 grainers:
204 = .239 BC
223 = .221 BC
Starting both at 3800 amounts to about 3 inches flatter and 3 inches less drift, at 500 yards. Not much to write home about there.
If Hornady's 204 40 grain VMax actually has a .275, then you get a noticeable difference at longer rangers. From my limited experience around 204s, I would agree with Steelhead--the 12 twist has some problems stabilizing the 40 VMax at times. But it is the only bullet worth throwing over a 223.
For wind, the 75 VMax trumps them all by a minimum of 3 inches @ 500. When loaded to equal pressures, it is about a 5 inch advantage. But who wants to shoot 75s at prairie dogs???
The 40 grain Vmax failed to stabilize in one 204 I worked with. Well, I shouldn't really say that. It may have been stabilized, but didn't shoot for crap. In another 204 with 22" barrel, I couldn't get anywhere near 3800 fps. In fact the 204 has been pretty disappointing when the chronograph was pulled out, IME.
The 75 Amax is brought out when p-dogs over 500 yards are on the menu.
