The extra 0.2 grains is well within what a lot of measures can throw. If you have no signs of high pressure or excessive cycling force, you should be fine.
In your case, the doubling and burstfire is a fired round followed by a slamfire(s), which we may surmise because your primer change stopped both. I have to say, slamfires in the .223/5.56 AR are unusual and you might want to double-check your firing pin protrusion is within spec on both guns. However, the most common cause is inadequately seated or high primers, and softer primers will be more prone to slamfire when seated less deeply than is optimal than others do. The CCI 450's and 250's will be more prone to slamfire than the #41's and #34. CCI changed the anvil foot tripod angle on those to reduce the sensitivity to military spec.
A properly seated primer will be about -0.003" deeper than the primer is when you just feel the anvil feet touch down on the floor of the primer pocket. If you can't feel where that happens on your equipment, a rule of thumb is about -0.004" to -0.005" below flush with the case head, on average. So, by whatever means you are seating the primers, you want to check that.
Many people believe a primer is best seated to just touch the floor of the primer pocket. This is wrong.
Primers need to be slightly compressed to work optimally. This was confirmed 30 years ago by the Naval Ordnance Station at Indian Head (NOSIH)
in this document, where you can see in the drawings at the end, their recommendation for reconsolidation (compression of anvil into priming mix during seating) as compared to Remington's wider (-0.002" to -0.006") recommendation. Going from just touching to proper reconsolidation typically cuts velocity extreme spread due to the more consistent ignition performance.
There are several primer seaters on the market that let you fix the depth of the seating action below flush.
This is one, but the increments of 2.5 mils are a little coarse. I've seen a bench tool advertised that has 0.002" seating steps, but I'm not finding it right away. Perhaps another member will post a link. You can use shim sets with the Sinclair hand priming tool to set how far its primer ram sticks up into the primer pocket when its handle is pushed 100% of the way in, and those kits can give you 0.001" resolution. The primer seating system that is part of the
Forster Co-ax press will give you fixed 0.004-0.005" below flush priming. (Note that the Forster Co-ax bench priming tool does not do this, though; only the press priming tool does.) The K&M Primer Gauge tool will let you individual measure cases and primers to get the primers exactly 0.003" reconsolidated beyond anvil contact with the floor of the primer pocket.
Or, you can try just seating hard.
"There is some debate about how deeply primers should be seated. I don’t pretend to have all the answers about this, but I have experimented with seating primers to different depths and seeing what happens on the chronograph and target paper, and so far I’ve obtained my best results seating them hard, pushing them in past the point where the anvil can be felt hitting the bottom of the pocket. Doing this, I can almost always get velocity standard deviations of less than 10 feet per second, even with magnum cartridges and long-bodied standards on the ’06 case, and I haven’t been able to accomplish that seating primers to lesser depths."
Dan Hackett
Precision Shooting Reloading Guide, Precision Shooting Inc., Pub. (R.I.P.), Manchester, CT, 1995, p. 271.
