I think this is a cool project and the video may teach you some new machining or fabrication methods even if you don’t want to make a retractable pen out of a couple of empty 30-06 bullet casings.
The pen looks like it’s pretty easy to make. The hardest part might be getting a couple of bullet casings without spending much. It looks like you can buy small quantities of used cartridges on Etsy for less than $10. You might want to also try asking nicely at your local gun shop or on Craigslist.
You may also have trouble finding the exact same pen if you’re in the US. I’ve never seen one like it and the designer is working in millimeters, which means he’s probably in Europe or Canada. But if you’re able to play with machine tools then you probably won’t have any problem finding a good substitute.
By the way, it looks like he made it on a 7×12 mini-lathe, which are pretty affordable and popular with home machinists. I’ve got one and really like it, and I still use it a lot even though I have a bigger and better lathe now.