Tinker, Hope I can adequately describe how I think this asymetrical crown stuff works. This is equally true for single barrelled rifles, BTW.

The gasses pushing the bullet down the bore have both mass and velocity. As the bullet exits the bore, the gasses violently expand. A perfectly symetrical crown allows the gasses to exert equal force on all 360 degrees of the bullet's base. If the crown is deeper, say at the top of the muzzle, the gasses will push the bullet slightly down as they exit, resulting in a lower impact point on the target. Same deal for right or left. You can adjust the POI on a SXS shotgun up or down by milling a slightly different angle at the muzzles.

About groups: My rifles start with the barrels milled perpendicular to the RIB at the muzzles. Then, I use a piloted 45 degree hand cutter to give a slight bevel to each muzzle. This is how the muzzles begin the regulation process. I use a 12 volt hand grinder with a carbide rotary burr to adjust the crowns at the range for fine tuning regulation. I insert a tight fitting bore swab about 6 inches into the bores to keep the grit out of the bores. Now, about the groups...there's no change in the groups during this process.

I now believe the concern over perfect crowns affecting accuracy is from asymetrical wear to the rifling for an inch or so at the muzzle due to cleaning rod wear. this will deteriorate grouping ability for sure. When the bores are perfect out to the crown, the asymetry doesn't affect the accuracy, only the point of impact.

Your rifle will certainly shoot to a different place if you mess with the crowns.

Otto