The last set I needed to shorten were exactly 1/8" too long which is, coincidently, the width of a typical lathe cutoff blade. The cutoff blade will cut the tubing fairly easy and will stop when it hits the hardened tip; this leaves enough of the tip to get hold of with a claw hammer or pry bar to pull the tip out, then shorten the tube more if required.

Like Bob said, the inside of the tube is usually reamed deeper than needed for the tip but check the depth before trying to press the tip back in the tube.