Aligned vents and created hex bolts

* Corrected barrel and bore lengths
* Added ring_barrel_diff to specify difference between the ring and the barrel, allowing the vents to be carved into barrel ring_barrel_diff/2, but still leave some of the ring intact for mounting
* Set vent diameter to 10mm.  Created second loop to only do 8 vents.
* Created hex bolts that appear to be correct size and positioned properly, BUT specifying height in hexagon (shapes.scad) creation is uncertain.