Good horns for making into powder horns are getting hard to find. If you are going to buy from Moscow Hide and Fur, Track of the wolf, etc., call them and tell them exactly what you are looking for. Ask about color, right hand or left hand carry, is the horn more flat then round or more round then flat, good for scrimshaw or not, etc. Hopefully the person on the other end of the phone will know something about what might make a good powder horn.
Randy Hedden