No problenms, clearly write "pen" on the custom label. Worst case if the metal detector and x-ray make them suspicious, they will open it and see it's a pen. I sent half a dozen from Canad to US without problem .
Canada Post prohibits mailing "replica munitions". Whether or not a bullet pen is a "replica munition" is up to interpretation - and I'm told that some postal inspectors say it is. You can mail a bullet pen to Canada, but there is a risk that it may be confiscated.