Shilen doesn't offer fluting, so their policy is that if someone else flutes the barrel, the warranty would be void. This just covers them in the case that the barrel is improperly fluted. Before I started buying barrels from them last January, I asked them specifically about their fluting policy. They reassured me that if I had a barrel that didn't shoot, and I had fluted it, they WOULD warranty the barrel. They went on to say that if, after inspecting the barrel, they thought that I perhaps fluted it too deep, they would still warranty the barrel and pass along their recommendations. Very stand-up folks at Shilen.
If you think about it, would any of the gun manufacturers warranty their barrel if someone had fluted it? No, of course not. They have nothing to gain, and everything to loose by endorsing any modifications to their "stock" product.
Since last January, I have purchased 138 Shilen barrels, fluted most of them, and had zero problems.