crimping for semi-auto & pumps

[allen]

I've searched previous threads on "crimping" but none really answered my questions.

My Lyman's #49 says to always crimp for autoloaders & pump actions, but doesn't say why.  Can someone tell me why?

I understand the spring pressure in pumps, but am not sure its that significant & don't understand at all why in autoloaders.

I've been shooting .30 Rem in a Remington mo. 8 (autoloader) & a Remington mo. 14 (pump) without a crimp as the previous manuals I was using (NRA's "Handloading" & "The ABC's of Handloading") don't specify a crimp. Of course this is a light round & I've had no problem with it.

I'm planning to start loading .223 for a Mini-14 so wonder if that light of a round needs a crimp.

I load .32 H&R Mag & .41 Mag both with a crimp, so crimping in its self is not an issue.

[Gutpile]

Because when the bolt slams forward on a semi it can push the bullet back into the case. I don't crimp for my AR and I have had it happen to me once. If there's another reason I don't know what it is but this is one reason.