I went with the #6 Heavy Bull Sporter, but I had them taper it to .820" at the muzzel to match the Vais brake diameter. I also had them leave the barrel at 28" (without the brake installed). I have a Weatherby .300 mag with an Accubrake factory installed, in all reality the barrel is only 24", they shortened it when they installed the brake to mantain the 26" barrel length, I'm about 100fps velocity short of the .300 is capable of. If the 28" barrel ends up being to heavy on the .338 ultra, I'll flute it myself. I run a 4 axis CNC mill where I work, wouldn,t take long to write a program and chuck up the barrel to cut the flutes into it.