Thanks for the responses ... including your very helpful speculation on the cause of this intermittent problem, Chuck ...
...
General, before I even saw your response, I already had the forcing cone kit loaded in my cart on the Brownells website (... well, actually, they now sell the chamfer cutter, extension rod, handle, and centering guide as separate items ... but they are all in my cart) and was just going to watch a few more videos before taking the plunge. The first video I watched was of modifying the existing forcing cone on a Ruger, and I was wondering if the tool would work as well when there is essentially no forcing cone to start with ... I take it worked OK for you?
That one video I have watched so far (... mannyCA, "The Accurate Revolver" series, on YouTube ...) mentions that the 11° which the Brownells cutter produces is optimum for revolvers shooting non-jacketed bullets, whereas various manufacturers produce their revolvers with different angles (presumably for jacketed bullets, or as a compromise) - e.g. Ruger use 5° and S&W use 8° ... so I suppose I can potentially use the tool to improve the forcing cone on other revolvers as well, since I shoot cast/swaged bullets almost exclusively anyway ...
This morning I got out the revolver again and, with the cylinder installed, aligned a chamber with the bore and looked down into it with a light ... the sharp edge at the rear end of the bore was quite visible all the way around the perimeter of the chamber mouth! No wonder this things shaves lead so much ... and spits so much of it back!
I could try smaller-diameter bullets too, I suppose ... but, in view of the non-existence of any forcing cone at present, I suspect that could only alleviate the problem somewhat, without eliminating it ...