Originally Posted by Rockfish Dave
The 6.5 CM has a taper of: .00724 per inch.
Reading Ackley books he even suggests even less taper per inch as a minimum (memory escapes me at the moment as to the exact amount).
Could your experience be because: The the brass used, rough chamber, or that you are using brass that has roughly twice as long of a case body?
I say this because I read everything I could find (including your impressive body of work) and compared allot of cases when designing my wildcats. I'd be happy to share my spreadsheets that I used for comparative analysis with anyone interested.
I probably use a bit more body taper then I need in my wildcats but there is a reason for it, Better to make sure you have perfect extraction at max pressures and a slightly dirty chamber then get sticky out in the field.
Ackleys wildcats were originally based on small diameter chamberings with most of his designs being on standard 0.470" diameter case heads or smaller. With this case diameter, you can get away with VERY LITTLE case body taper. Also, the shorter the case, the less body taper you can get away with.
The larger cases that Ackley worked with in his day were not loaded to the chamber pressures we are using today. Today, we load our large magnum chamberings to the same pressure as the smaller diameter chamberings.
The larger the diameter the cases, the baring surface to the chamber dramatically increases. Also, add longer case length to the equation and the baring surface really soars!!! Its because of these huge baring surfaces that its wise to use a bit more body taper.
In my testing, with my wildcat designs, those based on the RUM case work best with at least 9 thou per inch as a minimum and I prefer 11 thou per inch JUST to make sure that under all conditions, extraction is perfect with top end loads.
When I stepped up to the larger diameter Lapua parent case for many of my wildcats. I found that 10 thou per inch was absolute minimum and that 12 thou per inch was even better. With my Raptor wildcat case design with is simply a Lapua case stretched to 3.070" case length, I added another thou of case taper because of the added length of the case and increased baring surface.
With the Cheytac based wildcats, 12 thou is about minimum and 13-14 is much better for trouble free extraction in any condition.
The amount of case capacity you give up because of this added body taper is usually made up for in the ability of the case to have perfect extraction at higher chamber pressures allowing the case design to offer its full potential so its really a was in performance.
One thing most find also is that with new brass, there are no extraction issues of any kind but as the brass gets more firings on them, the brass gets a bit "harder" and has elastic properties in the case walls so extraction will tend to get a bit sticky with older brass and top pressure loads.
If you keep your chamber spotless, you will likely not have any issue but if you do see sticky extraction, you will have an idea what to do. At the velocity levels your seeing, I would think your pressures are plenty comfortable so you likely will not have any problems.