I ? The typical photos of primer flames that some experiments have taken tend to show 7-1/2 with more flame than some other SR primers--though as one primer manufacturer remarked to me during an interview a few years ago, flame length doesn't necessarily have anything to do with flame temperature, which is more important when igniting powder. (He suggested chronographing the same load, in the same rifle, using different primers as a far better indicator of primer "hotness.")
just some anecdotal information..
there is a NATO / OTAN paper " The Combustion of Solid Propellants" that discusses flame temperature, length and composition of various compositions in section 2.0 of the paper.
The paper seems more focused on the inner cone length than the secondary (visible) flame length (at all but the highest pressures ~100ATM), and specifically considers the two parts of the flame separately.
the pdf is 3.2 megs so I dont think I can attach it, but I could send it to you John.
looking at inner cone length might make for an interesting second look at primer flame photos....
Though I can't say for sure that priming compound fit the category of a "double base propellant"
