I have two questions for the technically inclined on this forum.

1. Why do some powder loads shoot better than others in a given rifle? I mean if all things but the powder load are equal, it should just be a matter of changing velocity. Yet, I know this is not true. Sometime 3 grains means a whole MOA in accuracy. Sometimes the difference between 4895 and 4350 (producing the same velocities) has an accuracy effect. I have heard random theories that mostly center around harmonics of the barrel or vibration. Can anyone shed some light on this?

2.Related question: why do most rifles seem to shoot better with near maximum loads?

KYNADOG,
I wouldn't call your questions simple by any means. For the first question, why do different powders have different results. Each powder has a different burnrate and as a result reaches it pressure peak at different points in the barrel, or at different lengths of bullet travel down the bore. Have you ever shot several shots from a prone position while lying on the ground? If you have, perhaps you've noticed how there appears to be a wave action in the dirt caused by the muzzle blast. This wave like appearance is caused by the barrel vibrating during firing.
Each barrel for many reasons, has a slightly different harmonics. These harmonics contribute to the accuracy of a load/barrel combination. It's much like hiting a ball with a golf club. If the "sweet spot" on the club face is the point of contact your get greater flight and distance with the ball, and less shock to the hands. If the point of contact is with the toe or heel of the club distance and accuracy goes to pot and the hands absorb alot of vibrations and tingle. With a rifle we are still looking for the "sweet spot" with the load.

For your second question, improved accuracy near max loads. I can't give you a good explanation, or even a poor one, for that matter. Some time ago this subject was addressed in a magazine. The author stated that this was brought about by advancements in powder technology, and was a fairly new development. Years ago the philosophy was that prime accuracy was reached with reduced loads. As I said, I don't really know. There was a time I accepted it as the individual rifle. I still am not sure. Good luck.

To answer both of your questions in a nutshell,,the less air space in a given load,,,the better off you'll be.Keeping in mind to stay within reccomended limits from a reliable info source for whatever loads you're making.

Your questions are a couple of the most UNanswerable issues you could have asked and your answer would probably fill a college text book that would leave us all about as ignorate after reading as before.

The answers above are as good a theory as any. My own theory is it's all controlled by the phase of the moon with a slight influence exerted by the color of my sox.

For question #2 the answer is that is usually where the most uniform velocities occur. Everything else being equal the least variation in velocity results in the smallest groupings.

Low velocity deviations are desirable but do not always provide the best accuracy and filling a case more does not always provide the best accuracy either.Each rifle has it's own preferences and you have to let it tell you what it likes by trying several load combinations to see what works best.