Steel bicycle cable jacket is tough, saw-defying stuff. The best way I know of to cut it is with one of Dremel's large-diameter (1 1/4"), reinforced cut-off wheels. Here's how to go about it.
1) Wrap a bit of masking tape around the jacket where you mean to cut it, and mark your cut-off point clearly.
2) Clamp the jacket end gently in a vise, and cut through the jacket with the cut-off wheel. Cut the jacket a little bit long.
3) With a belt sander/grinder, grind the jacket end squarely to finished length. Go easy so you don't heat up the jacket and melt its plastic sheath.
4) De-burr the end A/R and blow the jacket through with compressed air to clear out any grit. You'll end up with a nicely squared-off jacket end at an exact length.
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