Try upping the powder charge, if you are using a milspec primer you probably need to get closer to milspec pressures to make the primer cup fully expand into the cavity.
I've been using Wolf primers for a while now, and blow by hasn't been a problem with Lake City, PMC, Rem, or Winchester for me, using 24.4 grains of PowerPro 2000-MR which is slower than IMR4064. I would try 24 to 25 grains of 4064 under the 69 SMK.
Have you checked firing pin protrusion?
Machine guns are awesome until you have to carry one.