That does sound strange. Speer 75gr GD should be getting ~2500fps out of a 16" bbl. Considering that IMI is 5.56 pressure and a pretty hot load, Im surprised the other .223 loads were holding close to it.
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For reference I'm getting 2440 out of a 14.5" barrel and 2130 fps out of a 10.5" barrel with the 75 gr gold dot. They were grouping better than 2 MOA out to 200, but I have not tested beyond that. It chronos about 170 fps slower than mk262 out of the 14.5" barrels. 500 feet, 65 degrees.