Am I missing something?
The muzzles usually aren't deep enough (top to bottom) to drill two holes (or slots) on much of an angle and meet in the middle. It works best if the slot is a curve, so that it can exit higher on the muzzle.
I was thinking that I could cut it from the end, using X and Y coordinates to simulate an arc. The DRO has a routine for this. Then just make a fitting piece to seal the end of the slot, and held in place with a screw. Similar to the way many band slots have a little shim to fill the cut, but thicker and curved.