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OK, now I'm with you. I was thinking you were talking about increasing the bullet diameter as it was going down the bore. That was a new concept.
Pressure spikes like you describe are very well documented and reasonably well understood.
What they are not, however, is consistent. Sometimes you get a spike, sometimes you don't. As has been previously discussed here, that's why the 243 Win and 7mm Mag have been downloaded lately: unusually large pressure swings.
In the case described here, if I am interpreting the post correctly, there is a consistent plateau in velocities. This has also been documented repeatedly, but, in my opinion, it cannot be explained by varying obturation pressures. If pressure spikes were the explanation, it would be more random. Consistent plateauing of velocity is, again in my opinion, consistent with incomplete ignition.
Pushing beyond the plateau is rather risky: if you create a situation where all that powder does ignite early in the ignition sequence, it is equivalent to adding several grains to your charge all at once.
Fortunately, regardless of the cause (inconstent obturation or incomplete combustion), a more vigorous primer and/or a crimp will both lead to the same result: more consistency. JMO, Dutch.
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