Hope you have a milling machine; you want flat bottomed holes (or at least I do them with flat bottomed holes.)
Take a pair of V-blocks (one for the muzzle ring and one for the base ring) and shim the muzzle ring so the axis is horizontal. Then your only problems will be clamping the barrel so it doesn't move while you are cutting the holes and rotating exactly 180 degrees so the trunnions are opposite each other.
just finished, i sweated it let me tell ya,
im not a real machinest,
ive alot to learn when it comes to milling, i shake rattle and roll,
funny, did it just how you discribed, ha,,,
set a block at the rear, for a set point,
marked the front using the point of a tap in the drill head,
drilled hole, switched flattened it out with a mill bitt,
marked the table, rolled it to the front, spun the barrel, mark,
re-did all the above,
not bad for a beatup carpenter, wannabemachinestinmynextlife
johnp