Re: difference in primers?
Something I've found too be true (from fiddling with muzzleloaders and changing the physical attributes between the primer itself and the breechplug face) is that different brands of primers are actually different overall lenghts and vary by a couple thousands between brands.
This becomes especially critical when reducing the breechplug face to firing pin retainer clearance in an inline muzzleloader to reduce primer blowby and increase powder burn. Get to too tight (too little clearance and you get a slam fire when closing the action or change brands of primers and a taller primer causes the same scenario.
Which is why I stick with one brand (CCI). Cup diameter also varies a bit from brand to brand but the diameter varience is much less than overall height. Cup diameters vary a couple 10ths.
I've heard that one should always seat primers to the bottom of the pocket but I only seat my primers (in cartridges) flush with the casehead and no more. As widely proven, primers that are proud of the casehead can cause slam fires and rule of thumb is 1-2 thousands below the casehead, but again, I seat level with the casehead.
At least in my perspective, sticking with one brand and quality (benchrest, magnum or regular) goes a long way towatd shot after shot repetability.
I realize that with the component availability being what it is, that may not be practical but I would strongly suggest working up the load again, if you change brands of primers or the quality in the same brand.
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