Well I have been proven Wrong a few times but this isn't going to be one of them. Just for the record Chuck on the 28ga SAAMI drawing you posted we have a end of chamber diameter of .614". We then have a bore diameter of .545". There is then a cone connecting these two sizes having a 5° angle off the bore axis or 10° included angle.
Now I simply ask you to figure the length of that cone, I already have & know what it would be. Jut like to get your input as you seem to be saying all I have said is Wrong. I know you are capable of this simple calculation, would just like to here the answer though. "IF" it doesn't compute to be a "Modern Long cone" then an apology would be acceptable. If you can show me that it is a "Long Cone" then I will apologize.
Just to be sure we are on the same playing field I would consider a "Long Cone" would be a minimum of 3/4" long & really more on the order of 1" to 1˝" Certainly cones of 3/8" to 5/8" have been around for well over 100 years.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra