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The Factory Crimp Die is designed to full length size each round as it crimps according to Lee's literature. I've had the same thing happen w/.44Mag rounds when I crimp them too tight. My half baked guess is that like squeezing a tube of toothpaste in the middle and watching each end swell, the slug must be swelling in front of and behind the crimp. It also depends on the type of brass I use. Some of it does, some of it doesn't. Over-expanding the case mouth seems to cause the same thing.
You might try assembling a round w/no primer or powder at the "light crimp" setting, then tapping the bullet and checking for any movement. A tight crimp is a good thing, IMHO, but overdoing it just shortens the life of the brass from what I've read.
Can't help w/the 550b questions, as I don't use one yet. But having had a similar experience w/Lee dies in a Lee press, I believe it pertains to the dies and not the press.
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