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1) I have only used "double compression" to increase a 9mm case from 10.5 to 11 gr of Power Pistol. This was not practical, just wanted to see what it would take to blow the primer with a 124 gr Hornady FMJ FP at 1.169". The techinque could compress much more, but I have not done it.
Some powders like H110 and AA#9 have high bulk modulus [hard to compress]. They will bulge cases and bullets before they compress.
Some powders like AA#5 have so much power, that they can blow a primer or a case when half full and using a light bullet! No need to compress AA#5.
2) The Accurate Arms load for AA#5 and a 115 gr JHP is 7.0 gr 1.095", 4" barrel, WSP, 1192 fps
If I work up at 1.169", RP primed brass in a Kel-Tek P11, at 11 gr the primer pierces and the case starts to bulge in the shape of the feed ramp. At 11.7 gr the case fails, causing a secondary failure of some parts in the pistol, and some danger to the tester.
The difference of 11.7 gr failure and the 7 gr rating would be a safety margin of 4.7 gr.
If you loaded up some +P+ loads of 7.7 gr, the extra performance would come at the expense of reduced safety margin [4 gr safety margin].
One can only imagine that the big companies went over this with lawyers before they started selling +P+ ammo.
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