Thanks ndnchf,
Good pics, BTW! Looks just like the three rounds of Ten-X I've fired. You're right, the fired cases show a distinct shoulder. I imagine the Tex-X would chamber just fine in an original '76. I'm guessing that if the cases were sized with a "legacy" forming die, the cases would wear out quickly due to the constant fire-forming as illustrated.
Was going through my collection last night and found two Lyman moulds #515139. That should be a 0.515 bullet or there abouts (330 gr. RNFP). I'll fire up the pot, cast a few and try slugging the bore as you suggested. If the bore is indeed around 0.513, I should be able to load these as cast, correct? I'll probably end up getting a die anyway as that makes lubricating easier.
I have an interesting story about the first time I attempted to shoot this rifle. I bought a box of Ten-X from Cabela's and put off sighting in due to the high cost and time constraints. I went hunting with it anyway, and you guessed it, got a shot at a nice 6-pointer from my treestand. When I squeezed the trigger, all I got was a "click". I tried my best to chamber another round figuring I had neglected to chamber one in the first place. Round number two would not chamber, lacking about a quarter inch. You guessed it. A squib. That quarter inch probably saved me from a real bad day and taught a valuable lesson.
To make a long story short, the guys at Ten-X were great. They replaced my box of ammo and told me the malfunction was traced to a bad lot of Winchester primers. The problem was confined to a lot of 50-95 only and I was the second person to report this malfunction.
Thanks again!
Ed