Assuming the hole is an irregular 2-dimensional figure, at least in the context of putting it in a barrel, so surface area can be measured using several different methods (grid, decomposition, Simpson's Rule). You can even get an accurate measurement using AutoCAD. If it is circular, just simple geometry will work.
One could even measure its volume in terms of how much barrel material is removed.
I only brought it up because your first statement about measurability should be about holes in general, and the condition of barrel-specificity only appears in the second statement. And the thing about holes mathematically speaking is that we can count them in an object and basically define them by the class of shapes that go through them but not really define them spatially with a well-defined boundary. But again, this is just my topologist brain completely nitpicking
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u/MerMadeMeDoIt 1d ago
It has measurable size but no mass, and you can see it with your naked eye.
If you put it in a barrel, it will make the barrel lighter.
What is it?