Reply To: 3d printed
If you can print accurately enough, you could try printing sabots for sub-caliber projectiles. It would be great if you could use something like small nails, but I doubt they’d be stable. Maybe you could use 4.5 mm steel BBs in a 5.5 mm rifle. Or maybe you could get a bunch of military surplus flechettes and design your sabots around those.
For indoor target shooting, I wonder if standard 6 mm airsoft BBs are soft enough to be forced down a 5.5 mm barrel. Maybe you could make a sizer for them by sharpening a tube of the correct size.