Yeah, I realize that. I guess I am just wondering if anyone else has seen similar results? A lot of guys run long barrels on this forum and other run light weight mountain rifles and I would be interested in weather they are using a powder like Retumbo or 4350 and what barrel length/bullet combination etc?? Might save some guys money, time and barrel life.
To be frank, I don't think full burn is a worthy goal - or at least not one worth worrying about. As has been said, start with book recommendations and find what works for you in terms of velocity and ES.
I'm guessing that max velocity (with acceptable pressure) will probably coincide with lots of unburned powder because pressure to drive the bullet starts to fall off after the powder quits burning. Also, powder burn completion isn't an instantaneous event - rather, the burn rate slows down and then eventually stops.
From an engineering perspective, max velocity would result from constant max pressure and constant acceleration from start to finish. That isn't really possible with the powders we have today, but does explain why big cases result in higher velocities within acceptable pressure limits.
Personally, I like to look at case fill and velocity to choose the best powder to work on. I believe (but have no proof) that case fill is a better starting point when going after the highest velocity, lowest ES, and best accuracy.
Usually, case fill goes up with slower powders and down with faster ones. Look at the charts to see which powder gets closest to 100% at max pressure, then start lower and work up. This will usually also be the highest velocity other than compressed loads (which I avoid).
I don't even think about burn completion.
That all said, your observation of low ES might be interesting to think about from a purely technical perspective. I just wouldn't let something like that get in the way of developing a great load.
Just my two cents about a complicated issue.