Here is a question from the institute of silly questions.
I ran a batch of 338 Lap Mag cases through my Giraud cases trimmer.
Instead of being 69.2mm I set the machine wrongly at 68.2mm.
Is this a safety problem to load and fire these cases?
Will this affect the accuracy?
The 338 Lap Magnum cases( about a 100 ) are really expensive to throw in the bin.
Thanks in advance.

The rule of thumb is having one caliber of neck in contact with the bullet. Practically, if the bullet is held firmly, you're OK. The usual test is to shove the bullet into a solid surface, like your bench, and see if it slips. Boat-tailed bullets may reduce the contact area.

As a note, the 300WM has trouble making the one diameter rule.

A variance in neck tension may affect point of impact, but if all of your cases are uniform accuracy should be consistent. Benchresters may tell you to expect an improvement with reduced neck tension.

In short, if your bullets are held in place, you're good to go. I wouldn't expect 1mm (0.040") less neck length to have an adverse effect.

Thanks Winchester 69.
I appreciate you taking the time to share your thoughts.

You're welcome.