Hi Brian,

I found that the circle you used to extrude the tube is not planar or flat, see picture.
I'm not sure how you have drawn the circle but if you do need the end at an angle it's best to draw the circle say in the front view first then extrude it, do your shelling then draw a curve at the angle and position you want and Boolean Diff.

Unless the circle was supposed to be flat ? ;)



Hope this is clear.
Cheers