The Boers had problems with a batch of 7X53 mm cartridge cases they got among smuggled shipments of 7X57 mm ammo.
The 4mm gap was enough to let a bullet upset and expand enough that the rear of the jacket could seperate and shear off. When the next round was chambered the bullet telescoped inside the ring of jacket material jamming it in the throat and caused excessive pressures that Kaboomed a few Mausers.
A 1mm gap is unlikely to cause any safety issue, but if accuracy is affected you should correct the issue.
A case trimmer should have some sort of adjustment feature.