[DrMike]

Actually, 209 primers for muzzleloading are less hot than normal shotgun primers A regular 209 primer is hot enough to push the bullet forward before complete combustion of a load of blackpowder, which wreaks havoc with accuracy.

It is common to modify target muzzleloaders so that they use small pistol primers, the coolest primers available, but still more than enough to light blackpowder and its substitutes.

 

[DrMike]

Do NOT use 209 primers meant for muzzleloading in shotshells.
Stick to published loads. If you can't find the components you need locally, all are readily available online or via mail-order. Performing pressure tests, in your guns, next to your face, is beyond foolish.