- Weapon: Crossbow
A couple of things I've done in the past to replace missing pins with finishing nails:
If the nail is a little larger than the hole, take a drill bit that's roughly the same size as the nail and use it to ream the holes out. Do this by turning the drill bit by hand using a pin vise, pliers etc. OR Chuck the nail up in a drill and turn it at low speed while holding a file or sandpaper on it until you grind it down enough. If you do this, ALWAYS be sure the drill is turning away from you so if it grabs the file it's pulled away from you not into you.
If the nail is a little smaller than the hole, sand the nail down (just rough the surface up) and brush a coat of super glue on it. Wait for the glue to fully cure and try it. Repeat as necessary until the nail is the right size. Could take quite a few coats of super glue depending on the size difference.
If the nail's too long, you can cut it to length with a dremel, side cutters, end nippers, hack saw or any other tool of your choice.
You can employ either of these methods with a plastic sprue from a model as well provided it's strong enough for the application.