I don't really think that having a dome on a call makes much of a difference, although it may for some folks. From what I have found, the key, at least for bugling, is using a tightly stretched, uncut reed call ("tightly stretched" varying with the thickness of the latex material used in the call). Of course, the guttural sounds will/may need to be added with your voice.
For cow calling, the reed stretch may need to be a bit looser to achieve that "nasally" sound, and reed cuts may or may not make those sounds easier to achieve. Regardless, I don't think the dome is important, at least for folks who have a lot of experience using mouth calls...but may be for less-experienced mouth-call users.
