I just got my first big barrel (small to some of you) a Hern Coehorn mortar. I am going to have the vent hole EDM'd to play it safe...I break drills alot, and a machine shop with EDM owes me a favor anyway.
Anyway, does anyone have a clue how to set the barrel up to drill the hole?
The dimple cast into the barrel to start the hole is 9.25 from the muzzle. The barrel is 10.0 inches deep, and the liner appears very well centered and straight in the casting. I figure I would like a 0.200 hole that would enter the barrel bore at 0.400 from the bottom end of the chamber. The outside of the casting is a bit confusing.... hard to get a grip on the outside-to-inside relationship to calculate the angle. I am guessing about a 15-20 degree angle should enter at 0.400 and taper inward enough to guarantee puncturing even the a dinkiest powder charge bag at some point along the surface. I have measured it a dozen times and come up with different numbers every time.
Anyone else have any experience with this?
Thanks,
Rick
