What you're saying, Brad, is exactly what I first suspected, but the shell fired from the offending side drops right back into the chamber fully, and even seems loose, or at least the same sort of clearance as the non-offending side.
The indentation in the primer is visibly deeper than the non-offending side, and there is evidence of the firing pin dragging on the lower edge of the indentation.
A while later, I was using one of my non-ejector guns and experienced the same thing, so, to try and eliminate one or the other (Chamber vs firing pin) I took the extractor out, fired that barrel, had approximately the same resistance to opening, then poked the empty hull out with a dowel, with no resistance.
What bugs me is that when measuring the firing pin protrusion on the offending side, it seems no different than the side that gives me no issues. It just seems that that firing pin goes further into the primer than the other side. They are obviously in a rebounded position at that point, but for whatever reason, the travel seems greater on firing on the bad side. Can the body of the hammer actually contact the pocket in the frame if it were dry fired excessively? Worded differently, what stops the forward motion of the hammer when there's no shell for the pin to contact?

I'd probably have someone take it apart and give it a good cleaning internally if you haven't already done so. Just to be sure it's not a crud issue.

Steel bases are not only a problem with vintage guns. In my case, my one 12ga DH would have problems with even new brass based shells. It seemed more to do with the rim profile of the shell plus a tight chamber and shallow rim cuts on my barrels. Some manufacturers have a 90 degree cut and others have a slightly rounded rim to the body wall of the base. I'd notice that on some shells it was difficult to close the action. Upon firing, I could just get the barrels to slightly release from the breach face. Taking a wooden ram rod and going in from the muzzle and pushing on the spent shell would let the gun open fully. I had the rims cut slightly deeper plus I did a little filing on the extractor. It was real tight going into it's cutout.

Thanks Brad. I will send you a PM.
I like the cutting and polishing idea. Can I improve or damage anything with a brake hone?
Would a little grease around the lip help, at least to diagnose this issue?
In the mean time, I have 4 cases of federal shells on the way to give them a try. I burn up a lot of ammo shooting 5 stand at the Escondido Fish and Game so 4 cases goes fast.
Last week I brought this gun out of retirement and shot my best score ever after months of shooting holes in the sky with my auto-loader. In other words, I want to take care of this gun! I learned to shoot with this gun and it has been in the family since new. It handles so much better than my Major Brand auto-loader. There are no comparisons.

Matthew Winter
Escondido, Ca.
San Diego County
KI6TWW

send the gun to brad he is the best barrel man in these parts. the work he has done for me and my son is of the highest quality....like the saying gos "no brag just fact"

Matthew , all of the suggestions from fellow members have merit but I like to try the cheapest, less invasive approach. Try the Federals ....you might be lucky .

I had a Parker 12g. once that had a similar issue. It shot fine for a while and then wanted to open hard as the pins were hanging up in the primers just a bit. Actually, it was really only one side that was giving me issues. The end of the pin had gotten a bit messed up somehow. I just removed the hammer and reworked the end of the pin back to a round tip. No issues again after that.

Not to take anything away from what Brad was describing, but Brian, what you found is just what I was also finding, and just the same way; one barrel, but not both.