Thanks for the feedback guys. I have run through several different loads, though I have a lot more experimentation ahead. Roberts factory loads run through the gun beautifully and remarkably accurate.
My next issue to work through with this is primer backout... as I said I have lots to work through, and am a very newbie handloader. Here's the thing. Factory loads back the primer out about .010 and don't reseat them, which is as I understand to be expected and not a serious problem.
But my handloads, using 40-43g of IMR 4350 under a 117 SST with a Fed 215 Mag primer, with fireformed brass necksized only, back out primers about .010 too! I have a basic idea about firing pin/primer detonation setback, brass "sticking" to chamber, case stretching to reseat primer, etc, but I don't quite get this one. Should not a fireformed case fill the chamber pretty tightly, eliminating most of the headspace that allows primer backout, especially as much as .010?
I know Ackley loads are supposed to fireform just fine with factory Robts, and would assume this case to be the same, but should I try fireforming with a lighter load than factory standard?
As I said I have plenty of work to do yet, but any suggestions would be much appreciated as I work through this.