This project is a little different from your average ball machine. Oddly enough, I designed it with the K'nex gun-builders crowd in mind. Project Accuracy is a "ball-machine gun target". In essence, it provides a challenging new way to hone your aim. And the coolest thing is, it even keeps score for you so you can determine your overall average!
Since Project Accuracy works like a ball machine, there is a continual reload of the targets, so no manual reset is required. Turn on the motors, and the balls will be hoisted to the top. They will roll across a flat area with two exits: if you hit the ball as it passes, the ball will be thrust to the side, and take the back path (if the moving target is too difficult, there is an option for making the balls stationary for aiming and firing). If you miss, the ball will roll in a straight line and fall down the front path back to the lift.
The balls that are hit drop down the back path, activating the scoring mechanism (a 360^o ball arm with a peg to turn score counter one space). To get a tens place as well, getting the range of the scorer to 99, the balls are lifted up a second lift and run through an infinity switch, which only drops every tenth ball onto the second scoring counter, which is a mirror image of the first. At this point, all balls cycle back to the primary lift. (See pictures if this description is not clear enough.) By repeating this process, you can effectively count how many bullets you use (or how many in each mag, etc.) and compare that to the number of "hits" displayed on the scoreboard.
While this machine isn't flawless (thus the word "experiment" in the title), it is a neat project I thought others might find useful. Also, I did use a few parts in my machine that were originally designed by other Instructables members. I will note these contributions when I get to them in the descriptions below, and sincerely thank them for their work.
Hopefully you will find your targeting accuracy is better than mine! :-) Enjoy...

A word of caution: These are not full instructions!

Step 1: Ready, Aim, Fire!

The primary portion of Project Accuracy is the main lift, targeting area, and ball return for missed shots. The balls start by waiting at the bottom of the main chain lift to be hoisted up at regular intervals. Once they reach the top, they roll around a curve and onto the "gun range". This is where the machine becomes interactive. Once you see the ball coming, aim. When you are ready, shoot your favorite K'nex gun and (hopefully) the ball will spin backwards from a direct hit!
However, none of us are perfect, so if you miss the ball will roll straight across into the nearer hole, dropping down a chute to the ball return. That way the balls will keep on coming!
In easy mode, this ball return is bypassed, and of course any hit in either mode will send the ball to the back hole. While you congratulate yourself on your steady hand, it's time to see how this contraption keeps track of your hits!

Step 2: The Scoring Counters

This is the part of Project Accuracy which makes it so unique. The scoring counters help you keep track of how many hits you have made, and when compared to the number of shots fired gives you your accuracy average. The counters are mirror images of each other in order to fit, so keep that in mind if you build. Also, they are really a modified combination of Shadowman39's Skeeball Machine Counters and sandroknexmaster's 360^o Ball Arm. Many thanks to their inspiration and aid on this part of the machine! :-)
Here's how it works:
The ball falls through the hole in the floor (or pathing) directly above a 360⁰ ball arm. This ball arm is connected to a peg-like projection farther along the rod. When the ball falls onto the arm, it flips. The ball drops out underneath, the peg pushes a projection on the modified Skeeball counter, and momentum resets the arm. As always, if you don't understand the concept, make sure you read all the image notes, and see the pictures and/or the video in the introduction.

Step 3: Secondary Lift and "Every Tenth Ball" Control

After leaving the first ball counter, I could have sent the balls right back to the primary lift (in fact, it would have been a lot easier). But that would mean only nine shots before the counter reset back to zero, a rather poor range. So instead of ending right there, I added a second score counter to register every tenth ball. That way, the machine would register up to 99 hits, a much better range of numbers.
In order to work, the balls had to be hoisted back to the top via a second chain lift. At the top, a typical rocker path separator flips alternates balls between a path to the Infinity Switch (created by Sorunome) and a drop to the ball return. Because of this rocker switch, I was able to reduce the machine's length by half since the Infinity Switch needed to be only half as long (because now five balls bypassed it entirely, so only five slots would get me to ten).
When the first four Infinity Switch balls enter the switch, they simply drop down onto short paths leading to the ball return track. But the fifth ball (really the tenth ball due to the above) drops through a tire hole onto the second score counter, thus counting out every tenth hit by counting each ball that cycles through!
All the balls end up back at the primary lift at the end of their runs.

Step 4: No Finer (K'nex) Marksman's Tool...

And there you have it! Project Accuracy is like no other K'nex device currently built.¹ If it were to work without any problems whatsoever, the machine could stay continuously supplied by just a few balls, since they all cycle through just like in a ball machine. Even though the balls proved a little smaller and faster than I had expected, this difficulty can be remedied by adjusting the tilt of the target area floor or by setting in "easy mode" (see Introduction). I hope you enjoy honing your accuracy skills as much as I enjoyed honing my building skills!
Until next time...

¹I could not find any, at least.