Alamo city paintball sells a commercial delrin version of a "low blow bolt" for most markers. Definatly quality to thier products. Delrin bolts and accessories from Alamo City Paintball
edit: the face of these are still open with just a channel milled out on the bottom, but they are a custom shop so the owner may be quite willing to customize any bolts without the center boring. Seems to be a fairly nice guy according to a few people that know him.