I've been thinking of getting some of the "less hot" 410 primers! The subject comes up once in a while and sounds interesting!
The standard 209 primers have worked well for me in my Encore, but as I understand, they have way more power than required for reliable ignition, and the 410 primers aid in accuracy!
I don't really know for sure what you mean by getting "blow-back" in your Encore!
I "assume" you mean that you are getting smoke, soot, etc leakage around your primer and back into the action!!! I don't get any of this in my Encore.
If it was "SAFE" to do so, maybe you could back the breach plug out a 1/4-1/2 turn or so, until the primer was a tighter fit!
You'll have to ask someone with more experience than me, whether or not backing the breach plug out a little is "SAFE" or not!