Your gunsmith knows nothing about Contenders. The most common cause of misfires, besides the locking lugs not extending far enough into the frame, is too little headspace. This is usually caused by the reloader not sizing his cases enough; this prevents the action from closing fully and the interlock safety will block the hammer from hitting the firing pun fully. Sometimes there will be a deep indentation, but not quite deep enough.
Another cause is a gummy action. Did you know that you are not supposed to use oil in your Contender? Bet you didn't read that page in the owner's manual, but it is there. Use a spray gun scrubber/degreaser to remove all oil from the internal action parts and don't put any oil on them! Oils like WD-40 will dry and gum up, collecting crud until the interlock safety sticks and prevents a full-power strike on the primer.
I'd clean out the action and use factory ammo before I sent it back to Bullberry. You can check for correct function by using a primed case - if it goes bang, you've fixed your problem. If not, send it and the frame off to Bullberry. If it is the locking lugs, be aware that this same thing happens with factory barrels too, it just depends on the tolerences of your frame and the individual barrels.
