Off topic ramble, Primers, duplex loads, and achieving ignition and complete burning.
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@Stats Shooter - You mentioned 45-70. If you search the link below, you will find some load date for 45-70 using WC872 and a magnum primer (No Duplex powder stack needed).
I had not thought about Magnum chamberings and their powder columns. I think magnum primers in "tall powder columns" should yield more consistent results with the more appropriate magnum ignition. That may not "require" magnum primers for extruded powders, but I think magnum primers would give more consistent results.
This reminds me of "during the great shortage" I played around with very slow Ball powder WC872 recovered from 20mm Vulcan pulldowns. I picked up 8lbs of WC872 for $39.99 and 8lbs of WC844 for about $80 from the old Wideners. This led to three Options with the surplus powders.
Source info, albeit scant:
http://www.castpics.net/LoadData/Surplus/default.html
!. WC844 is equivalent to and replacement for H335, works fine, barrels get hot quickly. Serviceable for 65 gr and under. Also useful as is in 308.
II. One purpose of the WC872 was to mix it with carefully controlled portions of the faster WC844. In short a 872-50:50-844 mix was useful, predictable. Also steps of mostly 872 and 5 to 45% 844 were tried along the way. Nothing spectacular. But they all went bang and rang the steel plates. I tested these in 223 and only a bit in 308.
III.
[Not tested yet] My other purpose for the 872 was as a very inexpensive propellant for 308/7.62, using heavy bullets (180 & +). Ball powder AND VERY SLOW. My research on this lost out to competing priorities, But I have around 300 or 500 220 gr RN. I think these are too heavy for 308 duplex loads and I have been eyeing Savage 30-06's. To set them off, myself and others tried certain things, typically a 3 gr. booster of Unique, over the primer and under the column of cannon powder. That was actually the point of my ramble: With a very slow ball powder, yes, use magnum primers, but also be aware there are other potential ignition boosters. I am thinking I can get 2300-ish results for 30-06 with the 220 gr JRN projectiles.