30mm is 3cm diameter.
Which is for me 3 Fingers, as one of my fingers is about 1cm in diameter.
And my forearm is long enough.
From the ellbow to the hand O.o
Pinky tip to thumb tip when fingers are spread as far as possible.
No measurement available at work.
Your fingers are rather small then, I can't even fit two of my fingers side by side into the neck of the casing and it's significantly shorter than my forearm.
Next to a nickel(quarter did not want to stand on it's own today):
On it's end next to a sharpie and a nickle:
Forgive the photo quality, didn't have my other camera handy so I used my phone.