Case trim???

[mjsweims]

I was driving along the other day trying to figure out why I am case trimming bottleneck cartridges. First of all I use the RCBS hand trimmer with power drill attachment so I trim for total length. So my question is what good does this do, since I also full length resize first?
The only thing I can figure is that for crimping I get the crimp in the right place and maintain the proper COL. However, for plinking and paper I don't crimp. I only crimp for my hunting loads with cannelure bullets.
So if I have a long case the only difference it makes is that the neck will extend higher, and possibly there will be a minor amount of increased pressure. Also the volume will be slightly increased which can either increase or decrease pressure. Using COL to trim I have no control over proper headspace.
I can understand trimming off the shoulder. You'll have constant headspace and your fireformed cartridge will come to a constant volume.
So can someone please explain the logic behind trimming for overall length.

[bohoki]

if it is too long if you jam the bolt closed it can put an active chamber crimp on the bullet increasing pressure

you need to trim because resizing causes the case to extrude slightly

imagine taking a wad of gum you smash it with your hand now it is a fat short disk now you try to form it into a sausage shape it gets taller

[killshot44]

You need to measure and trim (if needed) AFTER you resize the case. The neck's length will increase during sizing.

The gun's chamber is cut to allow the casemouth, neck, shoulder and casebody to obturate to the size of the chamber when fired, creating a seal and directing pressure out through the barrel. If the caseneck is too long and extends into the throat you'll have difficulties closing the bolt and in getting a proper seal which can induce pressure problems and accuracy deficiencies.

The Possum Hollow trimmers index from the shoulder. Best way.

/ nice looking dog in the avatar