[Jack]

On once fired cases, I'd look at the sizing before I'd look at trimming them.
It's possible the cases are over length, but, not usually after one firing.
Set up your sizing die as Billy D has suggested, and make SURE you have some lubrication on the inside of the case necks- the expander button can definitely pull the case neck forward after sizing, as you withdraw the case from the die.
Evan, part of what you're running into, too, is the difference between a single shot and a bolt action. A bolt action has a great deal of leverage to help seat a slightly large case in the chamber, and a single shot does NOT have that leverage. If your cases are barely sized enough, you'll notice right away in a single shot action, maybe not in a bolt action.
Now, the idea of trimming once fired cases to make sure they're all the same length is a good idea, but, not because they're over length-it's to make sure they're all the same length.