Railgun - a 6 mm BB-steel slingshot

Be careful, don't aim at living organisms, and wear protection goggles during firing sessions.

This is a slingshot powered by latex elastics firing 6 mm BB steel. The central under-barrel handle keeps the two lower barrel parts together. The stock is mounted with brass inserts and screws. The first test looks promising, with a strong kick for the BB. I'll field-test it soon. Have fun.

Dev stage: Print ready. Constructive feedbacks are welcomed.

Assembly: ~15 min. Be sure to “clean” your printed parts from support or elephant foot extra matter. Gently sand the inner part of the rail where the ball is rolling and wax it if possible. Use the two barrel links to screw the upper rails together and the under-barrel handle to assemble the lower rails. Cut a 65mm long part in the cylindrical 3mm spindle, put it in the middle of the rails, add ~10mm washers on each side. Cut ~140mm elastic bands, tie a double knot on one end and slide them through the hole of each cover. You can then plug the rubber on each side of the rod. Then assemble the screws on the rails, glue the ball magnets inside the lower rail. Add brass inserts in the cross and lower back rail and screw everything together. Insert visors.

Printing tips:

• All STLs are replaced every time I update the model
• Expect around 20h print on a modern CoreXY, 500g of mix ASA|ABS/PETG/PLA (~14 euros)
• It should print on a 250x250 printer bed (I've a Qidi plus 4 at 305mm so couldn't check)
• Remember to print all rails on the same side on the bed to avoid having an unaligned central rail due to elephant foot or thermal expansion. For example, if you choose to print the lower back rail on its left side, pick the same orientation for all of them (see plates pictures)
• I used organic support everywhere
• I recommend ASA for the rails over PETG
• Print the trigger and side covers strong, with extra walls (6 wall loops, 2mm top and bottom shell thickness), infill 25%. The trigger response/stiffness will be tightly linked to these parameters
• Rails can be printed with 4 walls / 2mm, infill at 10%
• Stock and under barrel can be printed with 3 walls / 1mm / 10%, fill the part with the screws of the under barrel at 100% infill (with a local modifier properly placed, see picture)

Recommended filaments: 

• Rails (~240gr / 8h print) - ASA or PETG
• Trigger, Elastic covers (~24gr / 1h35h print) - PETG
• Stock, handle, visors (~220gr / 10h print) - ASA or PLA, eventually PETG
• Avoid ABS. tricky thermal contraction, elephant foot, etc.

BoM:

• A 6x2.8 cylinder N52 magnet
• Nuts & screws: 2*M4x50 (for Rear part of the rails), 8*M3x20 (for the linking of back and front rails), 2*M3x40 (for rubber band covers), two brass insert for the cross
• Rubber band: standard slingshot rubber band of 4.5mm in external diameter, 1.7mm in internal diameter
• 3 flat washers (one on each side of the rod and one for the aesthetic look of the BB insertion hole)
• Stainless steel 3mm diameter spindle/rod (which goes in between upper and lower rails and hook to the trigger. HAS to be 2.9-3mm in diameter for the trigger to hold/release properly)
• Some 6mm steel BB

Versioning:

• v1 - 3rd March 25: alignments problem with the rail junction, trigger too soft
• v2 - 4th March 25: new alignment system is a bit better but not good enough yet
• v3 - 6th March 25: no more magnetic junctions in the middle of the rail, but stiffer under-barrel and side inlets approach; the trigger is fixed
• v4 - 6th March 25: added a 10-degree angle on the cross, shortened the distance between trigger and cross by 15mm to have a better trigger control
• v5 - 9 March 25: added some clearance, simplified the rails linking, and reinforced the elastic covers. (the vertical through hole can be used to put some 3mm spindle leftover and tie a bunch of the elastic vertically to absorb the release shock)
• v6 - 10 March 25: added more clearances & print tolerance
• v7 - 16 March 25: Complete redesign of the cross ←> rails (got rid of the magnets). Redesign of the elastic stoppers with a vertical absorber (see picture) to lower the impact of the release. Added a Picatinny rail.
• v8 - 18 March 25: designed an alternative version of the elastic covers to double the rubber bands and the power.
• v9 - 274 March 25: a new trigger capable of handling the double rubber version. Print at 50% filling. Adjusted visor position. 

Todo:

• Maybe design a BB autoloader?
• A clay ammunition version (much more environment-friendly)