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According to one theory - since you have a fairly long leadpipe there, as is usually the case with rotary tubas, a more bowl shape for you. That theory is not the only one, but it's probably good enough if you need one.

What are you using right now?

These days, I mainly play my CC sousaphone, and my 1965, 186CC. For large ensemble playing (and home practice), I use a Giddings modified Baer mouthpiece on my 186. I asked Ivan if he could make the original Baer mp (cup) with the newer MMVI Baer rim. I liked my MMVI, but I felt the extra mass around the throat deadened the sound for me. I wanted a Baer with the newer rim and no extra mass around the throat. Due to recent digital technology, Ivan is now able to do this and he did. He even put my name on it.

Then, when I want to use my 186 in quintet or T/E playing, and on my sousaphone, I use the Jon Sass mouthpiece made by Ivan. It has the same rim as the Baer MMVI, but with a slightly more shallow bowl and smaller throat.

So, both of those mouthpieces work well for me on my 186.