I think what Mike saw was just the shadow around the edge of the mouth at the cannelure. Trick of the lighting.
When you first shoot the full power loads, look carefully at how much cannelure is exposed as you are loading. Check a couple of the remaining rounds after each shot until you finish the cylinder, looking to see whether the cannelure starts to come up. The main thing you are trying to avoid is a bullet nose that recoil brings out beyond the end of the cylinder and thereby jams cylinder rotation.
This happens in revolvers because the recoil pushes on the cylinder and the back of the cylinder pushes on the rim of the case. Inertia tries to keep the bullet in place while the rim is pushed back. It's sort of like "pantsing" the bullet. The heavier the bullet or the lighter the revolver the more likely this is to happen. More bullet mass has more inertia and a lighter gun recoils harder and more sharply than a heavy one. I have a friend who owns a super light weight titanium alloy snubby in .45 Colt. This fellow is a big guy with beefy mitts, but he said he can't shoot any bullet heavier than 200 grains in this flyweight or the unfired rounds will back out and jam the cylinder no matter how heavily he crimps and no matter how rigidly he grips the frame. Commercial 250 grain ammo does the same thing.
In magazine fed pistols, the opposite occurs, as nothing pushes on the rims of the cases in the magazine. However, recoil drives the front of the magazine against the bullet noses, tending to push them in. This is something a taper crimp is good enough to prevent, as the case has lower inertial mass than the bullet to drive against.
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