LEGO Machine Gun

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This page outlines a project that's about twenty years old, to make a ``gun'' that shoots 2-by-4 bricks at a high rate of fire. The later versions are vastly improved over the ``77'' model.

The Model 77 Machine Gun

When I was a teenager, one of the mechanical devices I built from LEGO was a machine gun that rapidly fired 2-by-4 bricks when the operator turned a crank on the right side of the gun. Unfortunately, the mechanism of this gun was both inefficient (the bricks came out at a pitifully low velocity, and couldn't hit anything at the same height more than a few inches away) and very damaging to the component bricks.

I drew up plans for this gun in September, 1977, not too long after building it. I drew the layers of bricks one at a time on graph paper; some layers are composed of plates marked with Fs for ``flat,'' and different parts of the mechanism are shown in numbered subassemblies. These plans are difficult to follow, but that's all right because you wouldn't want to build this model anyway. (See the much-improved 1996 model.) The basic idea was that the user would turn the rotor (3) counterclockwise by means of a crank. An arm of the rotor would push the upper end of a long rocker (2) forward. The lower end of the rocker would then pull back on four rubber bands whose other ends were looped around a brick near the forward end of the gun. These rubber bands provided the energy to propel the projectile. Meanwhile, as the rocker turned, it allowed a plunger (1) to be drawn back by a low-tension rubber band that looped around a tab on the plunger and a tab (6) on the gun's frame. This allowed one projectile to drop from the magazine into the barrel. When the rotor turned far enough to release the rocker, the four rubber bands would rapidly restore the rocker, pushing the plunger forward and propelling the projectile. This design worked poorly because of the inertia of these moving parts, friction with the plunger, and the resistance of the rubber band attached to the plunger itself.

The Model 95 Machine Gun

From December, 1994 and February 1995, I reworked the mechanism of the machine gun, replacing a sliding plunger and a long lever pivoted at its center with a small L-shaped hammer pivoted at its corner. I also changed the crank rotation from counterclockwise to clockwise, that is, moving forward at the top of the cycle, which seems to me easier to use, and added a hand grip at the bottom. As a result of these improvements, the gun can now easily hit targets a couple of meters away at the same height. It fires very rapidly, about 4 or 5 rounds per second. The main limitation of the gun is now the magazine, which extends vertically upward (for gravity feed), and in its current 30-round version is quite heavy and somewhat unwieldy.

I have a number of images showing the appearance and operation of the gun. I have not shown the full extent of the magazine, since this is really just a matter of how long one cares to build it. In actual use, the gun is held so that the bricks are turned 90 degrees from their ``normal'' orientation, that is, the studs of all the bricks in the gun point to the right (as seen by the user) rather than up.

The following images show the fully assembled gun:

For the following images, I removed several plates and other pieces that enclose the right side of the gun. This allows a better view of the moving parts of the action and the interiors of the magazine and barrel.

The Model 96 Machine Gun

During 1995 and 1996 I continued to make incremental improvements to the design of the machine gun. The current version (Model 96) has an almost entirely enclosed mechanism and a simpler rotor, and the handle has been moved forward (which I think gives better balance).

I am greatly indebted to the late James Jessiman, who used his LDRAW program to produce the following layer-by-layer drawings, which are the closest thing I have to assembly instructions. (The main difference between these drawings and real instructions is that pieces are sometimes shown before they can actually be added to the model, because the parts they attach to are in the layer above.)

Main assembly: layer 1 - layer 2 - layer 3 - layer 4 - layer 5 - layer 6 - layer 7 - layer 8 - layer 9 - layer 10 - layer 11 - layer 12 - layer 13 - layer 14 - layer 15
Magazine extension: layer 1 - layer 2 - layer 3 - layer 4 - layer 5 - layer 6 - layer 7
Magazine follower: layer 1 - layer 2 - layer 3

The following image is a PostScript rendering of the completely assembled gun, using Stephen Gustavson's LegoPS program. The rendering is imperfect because some pieces in the design didn't have good LegoPS definitions yet, and I provided only the fully assembled view, but I inserted comments into the source indicating which parts of the drawing code draw which parts of the model, why certain pieces were chosen, and what pieces are represented by the ``fudged'' parts of the drawing, as an aid to using this file as instructions. (If you are one of those very rare people who likes to hack PostScript, you can delete some sections of the drawing in order to view others.) There is also a GIF image of the finished PostScript drawing, which generally displays faster.

To use the machine gun depicted in the above images, install the rubber band (not shown) between the wheel and the fat part of the hinged striker insdie the gun (the rubber band should pass between the outermost two rows of studs on the hinged black plate, as in the photos of the Model 95 gun). Attach the magazine extender (the detached part at the upper left) to the end of the magazine (in the lower left corner of the diagram), or set that part aside if you prefer the short (7-round) magazine. Fill the magazine with 17 (or 7) 2-by-4 LEGO bricks, all turned upside-down relative to the rest of the construction, and finally turn the magazine follower (the small detached part at the upper edge) upside down and insert it in the magazine with the notch in the lower right-hand corner. Raise the gun so that the magazine points up (it's gravity-fed), grasp the red-and-white hand grip (at the right side of the diagram) with your left hand, and point the barrel (the red protrusion on the edge of the gun pointing away from you in the diagram) at your target. Grasp the blue cross-shaped handle with your right hand and turn the crank clockwise relative to the view shown in the diagram. Being careful not to jerk the crank off the rotor (the weakness of this joint is a minor flaw in the current design) I can empty the 17-round magazine in about 1.9 seconds, which translates to a rate of fire of over 500 rounds per minute.

Last updated Mon Sep 29 00:57:17 EDT 2003 .

From David A. Karr's LEGO Collection, by David A. Karr