Shooting Machine

    Archery shooting machines are a fantastic tool for bow tuning and evaluating arrow flight.   Several commercial shooting machines such as the Hooter Shooter and the Kwik-Shooter are capable of shooting arrows through the same hole at a target 20 yards away.  Unfortunately, these devices cost $500 to $1200 making them more appropriate for professional archery shops.   Looking at the drawings of these devices and watching a few YouTube videos of home-built shooting machines motivated me to try and build my own shooting machine.   My goal was to build an effective shooting machine for under $100 that could shoot arrows as well as the commercial machines.   This post details my design, materials, construction, and shooting machine performance.  The video below demonstrates the final product.   

Shooting Machine Video


Design

    I designed the shooting machine in Google SketchUp.  You can download the model for editing and viewing by searching "Archery" in the SketchPp 3D Warehouse.  Below are a few selected views from SketchUp.  The frame was built from 3/4" threaded black iron pipe.  This material is inexpensive, provides mass to the frame, and is easy to assemble using pipe fittings.   I used and inexpensive pulley system to draw the bow with a jam cleat attached to the blue slide.   The slide has two 3/8" unbolts attached in a vertical orientation with 1/2" white PVC covering the treads.   A thumb release fits perfectly on the PVC pipe and can be moved up or down to adjust the vertical aim.   I added hose clamps above and below the release aid to fix the release in a constant position.






Materials


    Materials were purchased at a local home store.   I have listed the major items below.   Total cost of the system was less than $100.   I did add a small laser to my bow to confirm that the bow was aimed at the same spot for each shot.   I took apart a laser level and built a mounting bracket for my bow.   I am also considering purchasing a laser that is designed for bow tuning.

  Number Description Unit Cost ($)     Total Cost ($)
 2        3/4" black iron threaded rod - 60" 8.00     16.00
 23/4" black iron threaded rod - 10" 4.00 8.00
 23/4" black iron threaded rod - 8"3.50 7.00
 13/4" black iron threaded rod - 12"5.00 5.00
 21" black treaded rod - 6"  3.00 7.00
 11" black threaded rod - 8" 4.00 4.00
 43/4" black iron 90 degree elbow 1.50 6.00
 23/4" black iron treaded coupler 2.50 5.00
 2                 3/8" x 6" Ubolts 5.00 10.00
 2          1/4"x2" Ubolts     1.50 3.00
 1     rope with pulley and jam cleat 15.00 15.00
 1 laser level 20.00 20.00
 2     1/4"x3" Ubolts to hold bow 1.50 3.00
   Total         $109

Construction Details

    The frame of the shooting machine is simply screwed together using the threaded pipe.   You can not screw together a closed object so I added two couplers 10 inches from one end.   The pipes screw into the couplers and the couplers screw together.   The slides for the release platform were made from 1" pipe that was drilled/sanded to make them slide smoothly over the 3/4" pipe.   I used a similar 1" pipe to create a bow mount.   


    The bow is attached to the 1 " pipe using clamps, the 1" pipe slides over a 3/4" pipe that is bolted to the frame.   
To make the frame square and strong make certain that all the pipes are cut to exact lengths.  Home Depot cuts pipe for free if you buy the stock in 10 foot lengths.   The most critical construction detail is to securely fasten the bow to the pipe bow mount.   


    The bow handle is not a regular shape so I made a wooden form (shown next to the grip in the figures above) that helps keep the bow straight and plump on the pipe.  A steel plate provides a backing plate for the clamps and also provides significant area of contact with the bow to reduce side to side wobble.  The bow can freely rotate forward and back, but not side to side.

Results



    Cabela's Stalker Extreme Arrows at 10 yards.   Arrows C, D, E, and F all shoot into the same hole.  Arrows A and H hit two shaft diameters to the right, and arrow B hits two shaft diameters to the left.   Any one arrow will shoot through the same hole.  Rotating arrow B 180 degrees results in a shot near arrow A.   As shown in the video at the top of this post, the fletch position for a given arrow is always the same, but this is not true for different arrows.


Same arrows from a different perspective. 

    Shot pattern for Beman Bowhunter 340 arrows.   Arrows 8 and 9 group together while arrow 4 is low and to the left.  Different color labels indicate replicate experiments.  Notice the effect of rotating the fletching from down to up.   These arrows have been shot hundreds of times and have been reflected at least once.  The precision is one inch at 10 yards and should produce two inch groups at 20 yards.   Not bad, but I would want to find a set of four matched arrows before deer hunting in Maine.  This is why I built the shooting machine.

    I tried the experiment again with more arrows.   The results are shown below.  Arrows 1 and 4 hit the center of the pattern.   Arrow 5 is low and right while arrow 6 (9) is low and left.  Consistent with the experiment above, arrow 8 is hight and slightly right, but not as far right as I would expect.  Each arrow was shot twice and entered the same hole.  All of the shots from all of the arrows would fit inside a circle the size of a quarter.  At this level of precision I have to be very careful not to shift the target slightly while removing arrows.  I am now being careful to shoot the entire quiver of arrows before repeating shots to avoid systematic errors in arrow placement due to target shift.  Only after the second quiver of arrows fall in the same holes can I have confidence in the results.   I can't wait for the snow to melt so that I can try the machine outside at 50 yards.




    Stay tuned for more experiments in the next few months.   The first thing I want to do is measure arrow speeds as a function of flight distance.   With the shooting machine I don't need to worry about shooting my chronometer with the arrow.  It will also be interesting to compare paper tune patterns for the different arrows.   I never seem to get good results with paper tuning and now I may have better success.  I plan on placing the tuning frame at several distances between the bow and target.   It will be interesting to see if the tear patterns change systematically with distance.

The outdoor update.

In January 2013 I moved the shooting machine outdoors and mounted it to a wooden sled made of scrap 2x8" planks.


The machine shoots very well at 35 yards as illustrated by the my second Robin Hood.


Not all shots are that reproducible, but I have used the machine to great advantage in evaluating arrow performance as a function of fletching, weight, and shaft composition.


Please see my post on the fletching shootoff for more details.


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