Thanks for the insight, Unclenick. I do agree that starting load work up at 10% below max should be the norm. Let me give you a bit of my reasoning for starting at 38 grs., though.
I had already done a complete load work-up from min to max listed in the Sierra manual for the 55 gr BK. I new that 38.5 gives me the best accuracy with the 55 gr bullet and I had a measured velovity with that charge.
In switching to the 50 gr BK, being 5 grs lighter, I figured that 38 would be a good starting point.
I wasn't planning on going up any further, as 4050fps seems pretty much at the limits of safe velocity (pressure) in my mind. It's pretty fast!
Anyway, that is the explination of my thinking with this "experiment". I think I will mess around with some different loads between 39 and 40 grs 4064. Maybe do these in .3 gr steps? See if the barrel sweet spot is in there some place.
BTW, I have also read on this forum and in other places that small variations in powder charge, like .3 grs, often don't make much differance at all in accuracy and velocity? In other words, if a group of 100 rounds is loaded with charges ranging from 39.8 to 40.1 grs, the rifle and shooter will not tell the differance in POI and velocity measurements? Is this true?