The perfomace of the 204 is pretty inpressive as are those speds across a chronograph. However the 204 is a tad probelmatic for shooting the bigger/longer bullets thanks to the magazine restriction. Also the brass is a tad expensive when you can find it. I have three in bolt guns that shot lights out.
That being said in the AR I moved to a Black Hole 20 Practical built by RPG last year and with the use of a simple 223 bushing die, one bushing and virtually free 223 barss I am GTG. Velocities between the two are virtually identical and the PD's I shot, shooting them side by side, sure can't tel the diference
I had been shooting a Shilen blanked 20 Tactical prior to getting the Practical. All three together in velocites makes a ballistic triad in the field. The only failing with it, IMHO, is that you do need a set of dies that are more expensive to get it set up. Once again the brass is so available as to be laughable. Case forming is not hard if you use the preoper dies and take a bit of time at it.
I actually make my 20 Practicals and Tacticals on my Dillon 650. An hour there makes plenty of cases and if you lose one you don't cry as much as with the 204.
Ritchs Precision Guns is using Black Hole barrels in his uppers in 204 and the 20 Parcticals as I type this. The price is reasonable and if you desire just a barrel of your custom profile ordreed up he can usualy cut the wait time dramatically.
Greg
Here is my 223 based family that I am shooting currently. All but the 204 are in AR's.
17/223 20 TAC 20 TAC 204 223 6X45