Why more powder for 223 than 556

[1168]

200 fps is a substantial spike. That can definitely cause a high flyer during a match. If the load is already on the high end of acceptable chamber pressure, that can cause an unpleasant surprise.
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We’re talking about a 68gr load that occasionally exceeds a max 62gr load by 180fps. Its a miracle that it hasn’t Paul Mausered me.

[jsbhike]

Factory ammunition fired in factory rifles that have not been modified?

Google "hornady frontier kaboom" and you will still get lots of reading material.