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These are all pictures of the spar after the final gun riveting was completed.  I had been very worried about riveting the spar.  The bolt method is not as repeatable or reliable as the gun proved to be.  It is cheap, and I agree that is will work, but if you have access to a 3x gun, from some friendly neighborhood RV builder or EAA chapter member, abandon the bolt method and back rivet.  The bolt method IS back riveting. When using the gun with a back rivet set – with a collar – it is very difficult to screw up.