I have did some pretty extensive testing with cases of Blackhorn 209 and all types of ignitions (#11, .25 ACP, several 209), with several custom and experimental breech plugs over the last two years. 11 different 209 primers, with primer carriers, Full Plastic Jackets, DISC's, bare 209 primer pockets, o-ring lined primer pockets, concave, flush, convex, with and without vent liners, you-name-it. This would include the .25 ACP breech plug with standard small rifle primers, and small rifle magnum primers as well.
My findings are that the .25 ACP will ignite BH209, but there is a slight delay with small rifle primers. With the small rifle magnum primers, I can get better ignition with saboted bullets in moderate temperatures. When the temps start to drop into the colder ranges, the ignition starts to wane as well. I have had delayed/hang and mis-fires, even with the magnum primers in just +40 F degree temps with some bullets.
It gets to the point that I would never trust it on a hunt, unless I was starving, and I had no other option. In Michigan, the December Muzzleloading Season can find you with temps from -20 F (or worse) to +30 F (or warmer), depending on the year. I think you will find that Cecil himself would not recommend the .25 ACP for hunting with Blackhorn 209.
IMO, the .25 ACP breech plug is not reliable enough to stake a hunt on, even with the hotter small rifle magnum primers. If you are just punching holes in paper, then that is a different story. With Blackhorn 209, the hotter the primer, the better the burn. This also gives you more mileage (FPS) out of your powder, and less residue left over in your bore. One of the ingredients in Blackhorn 209 is sulfur, this can be corrosive, and even more so with substandard ignition. Another reason that Western recommends the hottest Fed 209A and CCI 209M primers. I too would recommend these 2 primers in most OEM 209 breech plugs.
