I got tired of waiting for other people to design a 3d-printable Big Gun cannon so I made one myself. This cannon uses BigGunJeff's 3d-printed Torpedo Cannon as a base. Download parts and instructions for them here:
The magazine consists of four layers.
Bearing layer, top right. uses one 3016-2RS bearing, 1/2in ID x 1_1/8in OD x 5/16in thick (6384K49). The coupling is a K&S brass tube 8139, 1/2in OD from Amazon. Two #4-40 x 1/2in setscrews (92311A110) to adjust feed, up to four #4-40 setscrews to hold the bearing in place, and four #6-32 tapped holes for assembling the other layers.
Manifold layer, bottom right. curved to help redirect air towards the breech. Barrel spacing 0.6in
Breech layer, top left. ramps neatly fit 7/32 rounds, but can easily be drilled out to 1/4. There is a 1/4in wide x 1/8in deep lip, to allow installation of larger magazines on the same size base.
Magazine ring layer, bottom left. Single layer ring for 7/32 or 1/4, 3.5in OD. I will update with other diameters as I produce updates.
Riser piece, center. This piece is for holding pieces of K&S brass, brake line, or other material for the risers. Or you can install the barrels directly into this piece, if you don't want barrel depression.
lid: not designed yet. I can't 3d-print transparent materials, so this will have to be laser-cut or manufactured by hand.
Magazine height 1.5in
Magazine outside diameter 3.5in
cannon base height 1.25in
cannon base length 8in
overall height, 2.75in
accumulator volume 4cu.in.