Yes, there is a reason, and what it is, is all the phone calls and emails from disgruntled CVA owners, because their rifles wouldn't reliably shoot Blackhorn powder. Not knowing what to do, but trying to satisfy, Western Powders made the obvious recommendation.. use magnum primers.

Later they learned the problem was the breech plug. So they came out with a better breech plug design. Outwardly the Western Powders breech plug looks nothing like the Omega/Encore breech plug, but inwardly this new QRBP has virtually the same dimensions as the old Omega/Encore breech plug. It has the same diameter, and length flame channel, and the same flash hole.

Magnum primers never were required when using the old Omega/Encore breech plug, nor are they required now. Nor are magnum primers required when using the Blackhorn QRBP plug.

Some myths seem to die hard, and the myth that magnum primers are needed to ignite Blackhorn is such a myth.

Ignition issues when using Blackhorn are not primer issues, rather they are breech plug design issues.

It is a simple matter to slightly modify the old CVA breech plug, and the QRBP, so that they reliably ignite Blackhorn. There are several old thread on this forum showing solutions that are 100% reliable igniting Blackhorn. None of these solutions require the use of Magnum primers. These solutions use standard shotgun primers to ignite Blackhorn in temperatures as cold as -15. None of these solutions involve drilling the flash hole larger, and shortening the life of the breech plug neither.