Believe the target podcast #92

[Shadow9mm]

This is with Tim Sellars of Sellars ballistic technologies
It goes over some really cool stuff on barrel harmomics and "positive compensation.

as best I can understand it positive compensation is tuning your load so that, as velocities decrease the bullets hit higher to compensate for the velocity loss and reduce vertical stringing. making sure the barrel is moving upward in its vibration as the bullet is leaving the barrel so that slower bullets exit higher.

typically if you used a ballistic calculator and put in 2 different velocities you would get 2 different points of impact, 1 high and 1 low due to the increased fall of the bullet due to its lower speed.

the testing process basically constitutes to firing 6 shots, 1.0g of powder apart at a horizontal line going from right to left going from min charge to max charge

when reading the target normally, left to right you will have your fastest charge first. as you progress right you have lighter and slower charges. the bullet should be rising as you progress right. at some point the bullets will begin to drop off significantly faster than it was rising. any point where the bullet is rising is a safe area. stay away from where the bullet drops.

in theory, since the bullet is rising as the powder charge decreases, it should negate changes in ES/SD of a normal load and keep the POI the same, compensating for the variation within a normal load.

Really interesting concept.

https://www.youtube.com/watch?v=xvtllOH_8BE

 

[Unclenick]

It is an oldie but goodie. Randolf Constantine wrote about it (though not with the term Positive Compensation) in Precision Shooting back in the '90s. Varmint Al discusses it some, too, in his barrel tuner analysis information, where he refers to the opposite effect as "negative compensation."

The one limitation positive compensation has is that it tunes to a specific distance at which the trajectories of the two muzzle velocities cross over. This is part of what frustrates people about Audette ladders. They change optimum load with range. Not so much at shorter ranges, but at long ranges where the bullet is falling fast, you can have the positive compensation tuned to 900 yards and have a 2650 fps MV and a 2600 fps MV 175-grain MatchKing impact at the same exact point by having an additional 0.0273° of muzzle elevation when the slower round exits the muzzle, but they will separate vertically by about 21 inches when they get from there to 1000 yards. So you end up wanting Audette ladders for each range and ammo tuned for each range to eliminate that. Or, you start with something faster and higher BC by a good bit.

Therein lies one of the pluses of having a load that is not only tuned to your barrel but tends to have a very narrow velocity spread. You then need less of that sort of range-specific compensation.