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The Retreat Challenge: Testing The Poor Man's Chrono

The tools of the trade; cans, BBs, chrono, gun and a notebook.

Introduction


If you’ve been around airsoft for any length of time, you’ve probably wanted to chronograph the velocity of your guns, but have found the price of a chronograph prevents you from doing it. So, you decided to ask on a forum how to chronograph your gun without a chronograph. You were told to search for the “Poor Mans Chrono”. The poor mans chrono is simply a scale of the velocity required to puncture a Coca-Cola can in several key areas.


In this article, I will explain how and why the “Poor Mans Chrono” works, and why it works so well. All the testing was done by me, in the interest of proving Redwolf Airsoft's "Poor Mans Chrono". This article is not meant to end debate on "The poor mans chrono". Rather, I am trying to prove if TPMC actually works.


Using The Poor Mans Chrono


To properly use the chronograph, you must first obtain an empty Coca-Cola can, and a copy of the Velocity chart.


Your Coke can must be in perfect condition, free from dents, dings, wrinkles, punctures, or creases in the metal, or your results will be flawed.


Load up the gun to be chronoed with some .20-gram bb’s. If you use .25-gram BBs your results will be flawed; I’ll explain later. Now align the muzzle of your gun 1-2 inches from the surface of the can, any closer or further, and your results will be off. If you are shooting through the side or sides of the can simply stand it up on a level surface. If you are shooting at either the top or bottom of the can, lay it on its side. Lastly, do not hold the can, this messes up your results too, again, I'll explain later.

This picture illustrates the proper setup for the Coke can chrono. Notice the distance between the muzzle and the can.

This picture illustrates the proper setup for the Coke can chrono, to chrono your gun using the top or bottom of the can. Notice the distance between the muzzle and the can.

Now that you have everything lined up, fire and see what your gun can do! In order to accurately gauge your velocity, your shot must pass through the target are of the can in one shot; if it doesn’t, you’ll need a new can. Near breakthroughs and "It almost went through!" won't cut it here, that BB has to be on the other side of the target.

340fps penetrates both sides of the can.

There are six ranges of penetration on a Coke can. In order of difficulty they are:
One Side…………….290+FPS
Both Sides…………..340+FPS
Bottom Center………385+FPS*
Bottom Edge………...450+FPS
Top Center…………..470+FPS
Top Edge……………500+FPS


*According to RedWolf Airsoft this area is penetrated at 430FPS. However, in my testing, and that of others, we can consistently penetrate this area of a can at a minimum of 385fps.


Why Don’t .25-gram BBs Work?


A .25-gram BB can’t be used because they have more force on impact at a lower speed than a .20-gram BB traveling at a higher speed. Meaning simply that they can puncture the aluminum with less force.


Why use Coke cans?


Coca-Cola cans are used because of their near perfect uniformity. All Coke cans are made from two pieces of aluminum; the extruded cylinder and base is one piece, and the stamped cap is the second.


Coke cans are designed to weigh exactly 0.48 ounces, and the walls of the can measure only 0.003 inches in thickness, less than two sheets of standard paper. Coke cans also have the ability to withstand a force of 250 lbs being set atop it in a column. Try to visualize two coke cans one on top of the other. The can on the bottom is empty, while the can on top of it weighs 250 lbs. The bottom can would not be crushed, because the weight is perfectly distributed.


Another reason for using a coke can is because of its shape. The walls of the can are a circle. This shape is naturally strong, and provides just the right amount of resistance to be used as a chrono.


Why Can’t I Hold The Can?


When you shoot at an unsupported can, the can moves with the BB, thus it absorbs some of the impact. This absorption makes it harder for the BB to penetrate. When you hold the can or brace it, the BB encounters less resistance. On a safety note, why would you hold something while you are shooting at it anyway?

385fps Enough to scare most players.

Will Other Types of Cans Work?


Possibly. While we didn’t try any cans other than Coke, It would be safe to say that any beverage canned under the authority of Coca-Cola would have similar cans. If you use a can made of tin or steel, like a coffee can, the BB will bounce back and you will learn the painful truth of the saying, “You’ll shoot your eye out!”


Important Stuff:


The BBs used for my testing were Airsoft Elite .20-gram BBs.


The chronograph used for my testing is the ProChrono Plus by Competition Electronics Inc.


The velocities given are approximate; your Coke can may be slightly different.


Seriously, don’t shoot at coffee cans.


Information Taken From:


Projects.olin.edu


Hosford, William F. and John L. Duncan. “The Aluminum Beverage Can.” Scientific America Sept. 1994: 48-53.


Redwolf Airsoft, Bite the Bullet 2000-02-10, Bite the Bullet

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