No flex at all, what the grooves in the bullet sidewall do is remove bearing surface(the sides of the bullet that contact the rifling) and that lesser amount of contact/friction decreases the pressure because the bullet is easier to push out of the barrel. The bands can allow a heavier bullet to be shot faster than a lighter bullet that does not have the reliefs cut giving it more contact/friction with the barrel.
Notice the contact points from the rifling on this bullet and see how they only contact the high spots.... the groove reduce contact with the barrel = lower pressure which is longer barrel life. Or you can have equal barrel life but be able to shoot the bullets faster.
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Thanks for the response.
To report a bit, I contacted Barnes about the bands and the LRX 212gr vs. the 208gr. They pointed out that the 212 grain has a smaller section of the bullet that rides on the barrel. that section is near the back of the bullet. The forward portion of the bullet is a slightly smaller diameter (not the nose or boat tail, but an actual long section of the center of the bullet) so it doesn't ride the bore and cause a pressure increase, according to the manufacturer. See the image they sent me with my own drawing on it. The green part is slightly smaller and the pink portion is what rides the barrel, giving it the "bore rider" name. Total surface area riding the barrel may be similar to the 208.
The also informed me that having the smaller section of the bullet that is actual 0.308" means you don't have as much adjustment in seating depth. Also, the 212 was originally designed for 300 Norma Mag. But can work in the 300 PRC. In some cases it may become a single feed cartridge, but I have some that fit my magazine.
I don't know what the expansion to the second section will do to the aerodynamics. I studies some subsonic aerodynamics in graduate school and subsonic it wouldn't be a problem, but supersonic flows are a different beast.
I also wonder whether or not the smaller portion riding the barrel on such on long barrel can fully control the concentricity of the bullet axis. The distance from the front of the bearing surface to the rear on the 208 is over 2x longer than the bearing surface on the 212 which means controlling concentricty of the axis would require less than half the force, and less than half the dimensional accuracy. So, would the tip of the 212 be vulnerable to wobble?
As LowDownDirtyBugger says I got some 212's from Choice Ammunition and when I can get primers I'll start learning to reload and try the 208 and 212 against each other.
Thanks again for the response and lessons learned.