I believe it was Dan Pharris that had a good solution when this came up before.
Get a steel rod long enough to reach the bottom, and put the slightest bend in it. Upset a tooth on one edge of the rod, and use it as a scraper to cut the hole lower. I've done this a couple times, and it worked well.
Tapering the ramrod helps, as does making a cut in the lock bolt to facilitate passage. This can be done just on one side, that will require the ramrod being removed for bolt removal, or if there is enough clearance, chuck the bolt up in the drill press, and make a divot completely around the bolt waist. Ramrod still needs removed to take out the bolt, but easier to index the bolt back into position.