Trimming cases....What happens if you don?t?

[lazyengineer]

If too long it might not chamber, Worse than that is the rim area of the case neck cannot expand as it is in the throat and can greatly increase pressure which is never a good thing

This.

I will say, with sharp shoulder rounds like Grendel and Creedmoor; it's less of an issue. Of all the many cases I've run through my trimmer in these two 6.5 flavors; almost none actually trim anything; and almost always are below the "trim to" length.