Reynolds357,
The real test is to take your chronograph and check to see which primer produces the lowest SD and ES with your bullet and gun with this powder. That will be the one producing the most consistent ignition. The problem with less consistent ignition, in addition to the greater threat of getting a round that squibs out, is that it is associated with variation in exact time between firing pin strike and the bullet exiting the muzzle. This irregularity can be up to tens of milliseconds when it gets really bad, so it can have the same effect as having varying lock-time. It exaggerates the effect on group size by any imperfection in the shooter's steadiness and follow-through.
Shadow9mm,
Sounds like some of your bullets were being unbalanced by either gas cutting on one side or simply distorted by bullet tilt or by the bore engraving it with rifling marks unevenly. If there is any way you can recover some of the bad-behaving bullets, you should be able to see the cause.
Empirically, I have found shooting cast lead 0.002" over groove diameter often groups better than shooting lead 0.001" over groove. This depends on chamber dimensions, but if your mold will allow it, it is worth a try.