I can't speak to the .204, but yes I do have a .20BR. Mine has a 26", 12 twist barrel, and I can push the 40 gr. bullets about 4100 - 4150 fps. My 12 twist doesn't shoot the 40 Vmax all that well, shoots the lighter 32's MUCH better (at 4450 fps). It SHOULD do fine with the Berger 40's, and now the Sierra 39's (which are slightly shorter than the Vmax). But I have not had a chance to try either of them yet.
I necked mine down in 2 steps. Using a Redding Type S .22BR die, just changing bushings.
The reamer that was used for my chamber was setup specifically to avoid neck turning when using Lapua brass. It's a .237 chamber neck. I can't remember exactly off the top of my head, but I'm wanting to say that my loaded round necks, without turning, are .234 - .235. Regardless, I did not neck turn them. Did have to trim them a good bit though. But that was the total extent of the brass prep. I did no primer pocket or flash hole work on the Lapua cases. Just necked 'em down, trimmed 'em and shot 'em.
As with any other wildcat or improved cartridge, fireforming accuracy really just depends on the quality of the chambering job and the concentricity of the fireforming ammo. Both were pretty decent for my .20BR, so my fireforming groups were as good as my final groups.
Truth be told, I barely did any load work at all with this cartridge. Spent a total of about 2 or 3 hours, and shot maybe 40 rounds, and had a load grouping right around 1/2" using the 32 Vmax at almost 4500 fps. Called it good, loaded up all my brass and went prairie dog shooting. Never have circled back to do any more load work.
I'm planning on a .20-250 for next year.
- DAA