The square corners are on the flat and the recessed chambers. Both can benefit from chamfering. What you want is a small amount of angle on the chamber entrance. You can use a countersink of 82 or 90 degrees. It is best to only put a small edge break on the extractor, then remove it to chamfer the cylinder the rest of the way. Too big a chamfer on the extractor will make it ride over the rim of the case instead of pulling it out.