I took roughly half of the human bone model from Profguy and made it into a foregrip for an airsoft rifle, mainly as a joke gift. If you print it with the right material, it's easily strong enough to be actually used.

I printed it from white ABS @ 260C then applied very brief acetone vapor smoothing to get a smooth and dirt-resistant finish.

The model is split into two parts to enable optimal printing direction. Print both pieces standing upright to get maximum strength. The base needs to spring a little and it will probably break if you print it flat.

Hardware needed:

• 2x M4 * 30mm socket cap screws
• 4x M4 washer
• 4x M4 hex nuts (preferably 1 self-locking nut)
• 150mm M4 threaded rod / "allthread" (or a 145mm-ish M4 hex bolt)

Assembly:

1. Put a self-locking nut and washer on one end of the threaded rod. Pull it through until the nut can't go in any further. If you don't have self-locking nuts, apply some Loctite or superglue.
2. Insert the threaded rod into the bone from the large end.
3. Slide the base onto the rod and index with the tabs. Check if the parts match up without any visible gap, sand or file some material away if needed.
4. Check if the rod doesn't stick out beyond the flat spot for the rail; if so, shorten it.
5. Put a washer and regular nut onto the rod and tighten firmly. Optionally apply some loctite to make sure it doesn't come apart due to vibration.
6. Place nuts into the hex recesses on the side of the base and thread in the socket cap screws from the other side. If necessary, open the channel with a 4mm drill.
7. Briefly apply acetone vapor smoothing to seal the layers. This also adds some extra strength by preventing the layers from coming apart. This also has the benefit of glueing in the nuts on the side so they don't fall out. If you choose not to smooth the print, you can use superglue to prevent the nuts from falling out.
8. Depending on the amount of smoothing and your exact print and rail dimensions, you may need to file out the rail angles on either side of the base to get it to slide smoothly onto your rail. It only slides onto the end of your rail, don't try to snap it onto your rail as you will likely break it.