Bob,
You'll find that in determining maximum safe working loads, the information so far given here on the forum is good sound advice. The cautionary statements about the tight tolerances of the FA revolvers not producing significant case head expansion is very true. Primers, especially in the Casull, are a poor indicator, as you are using rifle primers in a handgun case, the primer cups are significantly thicker and tougher than are handgun primers... consequently by the time you get pressure signs via the primers in the .454, you are well into dangerous waters as a rule!
What I would like to add to what has already been suggested is this: USE YOUR CHRONOGRPH!!!! if you don't own one, wait on other shooting frills, and go spend the bucks and buy a ๖ Chrony!
Why? Because that chronograph will tell volumes about your load status! In using both H110/W296 as well as AA1680 in your .454 Casull, yes, those powders thrive on pressure, and reach their most efficient burns at the top 5% of the pressure envelope. You can use this to your advantage in determining optimum loads with your chrono. You will notice a few things happening as you are cautiously working up a load using these powders. First, pay special attention to the ES when shooting strings during a load workup. In loads that are well below the upper end of the pressure envelope, there will be large, and sometimes very large ES readings. As you begin to approach the best efficiency threshold, those figures will drop, sometimes right into the single digits!
Once into the low double digits for your extreme spread figures, then go slowly, as the sweet spot will only be a few tenths of a grain one way or the other on a good load for peak efficiency. I rarely have tuned a load with these guns, where when the ES over the chronograph showed single digits, that accuracy wasnt' very good to superb!
Now, another essential tell-tale sign to look for is velocity increase when upping powder charges incrementally. At the low pressure end, you will see very little change in small powder charge increases with H110/W296 and AA 1680, then as you begin to appoach the pressure envelope that produces efficient burning of these powders you will observe significant velocity jumps with negligable powder increases. As you begin to increase charges, and the velocities are climbing, rather predictably with inch upward increment, you will also see the ES figures dropping. Somewhere in this progression, you reach a point where the load does what I refer to as flatlines, where powder increases give very little to no velocity increase for the extra fuel added to the fire. At this point stop! Absolutely stop! You have reached a point where the load has maxed out the efficient burning threshold of the volume of powder loaded. Any increase in powder at this point is only spiking pressures!
Now, after having taken detailed notes during your load development, you should see a pattern developing of velocity to powder in the form of progressive increases. This trend is most easily evaluated when you plot your powder vs velocity on an X-Y graph in a good spreadsheet program. This way you have a graphic representation of the velocity curve in relation to your powder charges. Now, if you also plot your ES figures for the same loads on that X-Y graph, that point where you have the lowest ES, with velocities that are proportionatly increasing with added powder will be your top, safe load that will also have the potential of being your most accurate load in terms of burning efficiency.
From this established point you can tinker with a few tenths of a grain one way or another to fine tune the load on paper.
Now, having said all this, it is very applicable to the .454 Casull since it operates safely at the high pressure thresholds was designed for!
I hope that this helps you, and others as well. This method is much more reliable than many of the other traditional "max load" tests.
Be safe and enjoy!
God Bless,
Marshall