Agreed. I might or might not have more to say with specific information as to specific loads to include primer, case, OAL, bullet and powder charge and whether a .223 chamber or a 5.56 chamber or something else entirely as some of the match or hybrid chamberings.
From the description my first question would be maybe a 5.56 load in a .223 chamber?
748 was more or less made for the 5.56 and works well in the .223. I have superseded 748 with TAC and other newer powders in the 5.56 and .223 ( I have a pair of 5.56 and a single .223 and I agree there is a difference) so I cannot speak as to current lots and there may be a lot to lot variation that great but I doubt it.
I certainly would expect a 55 grain Hornady and the Hornady book load of 748 to perform well.
If most loads do well I'd be curious enough myself to work up again and so rule out mistakes on the scale or powder measure because the behavior described is indeed most uncommon.
