THe first thing in my mind was, "I can't believe the man posted this description and teased us, but refused to show us pics!!!" But others have chimed in already.
Did he bush the firing pins and make smaller rifle pins? If not, that could be part of your stiffy problem. The big shotgun pins have big holes in the breach face and the higher pressure could have extruded the rifle primers into the firing pin holes a good bit. Just something to look at anyway. At any rate, I would definitely want bushed pins on the gun.
Has this smith chambered doubles in this caliber before? Just curious about how the reamer was ground. Many reamer makers grind reamers to minimum tolerance for accuracy's sake. But with the weaker extraction of the double a tight chamber could stiffen things up until the fired case let go of the walls. But if this guy has used the reamer on other doubles then he would know how it was working. You asked fo rideas, so there are a couple of half-baked ones. Plus I guess naything a .50-90 would be chambered in would have weak extraction anyway.
Can't wait to see your pics!!!
Oh, one more thing. You mentioned "stopper" several times, but always talked about shooting deer. Is this a deer stopping rifle like for tracking wounded ones that often jump up and run through thick stuff, or did you build it in case you hunt really big stuff one day? Just curious.
