Reheat the end of the tube trying to limit the heat at the very end of the capillary tube. Blow an initial pressure of air, then pause a moment, then blow till the end of the tube forms a nice bubble with an thin end. Using this process, you can achieve a thin end with progressavly increasing thickness to the sides of the capillary tube. Blow/pop open the end of the bubble in the flame.
Because it is not possible to find a cork small enough to plug up the inner diameter of a capillary tube, if you wrap some masking tape around the outside of the tube till it continues past the end of the tube and then pinch it off, it is possible to seal off many awkward shapes.
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