I did an experiment on my previous wooden dowel demonstration posted earlier in this thread. I routed a 3/8 cavity in the bottom of the channel to see how the revised geometry works out regarding the theory of the dust and dirt being trapped in between the barrel and the stock. The revised geometry still works well for the theory, probably better. I will post a photo when I get a chance. I also have another theory regarding the purpose of the routed grooves in the bottom of the barrel channel. The top edges of the routed grooves contact the barrel and I believe that they hold the barrel firmly in position. Much better than the barrel could be held in place if the routed grooves were not there.