Not everyone can afford a chronograph to test how much juice their favorite airsoft pistol or AEG has. Even after lots of modifications you wonder how much velocity you're really getting. Well you don't necessarily need to dish out several hundred dollars for a chronograph. In most cases, a simple soda can is all you need to gauge those major power milestones.
First, get yourself a regular EMPTY soda can and rinse it out. Coke and Sprite cans work the best as beer cans and other brands sometimes have varying thickness which can throw off your estimates completely. Through experience, I've found Coke and Sprite cans to be fairly consistent in strength around the world. There are three major surfaces on a soda can that represent increasing strength and thickness; the side, bottom and top, in that order. While the sides are quite easy to crease and crush, you'll probably be surprised that they're fairly resilient to abuse from an AEG using 0.2g BBs. A standard unmodified Tokyo Marui Next Gen SCAR will have trouble piercing even one side of the can. However, if you press down and flatten the surface a bit, you'll find it fairly easily to penetrate, even with a spring pistol shooting at 0.3J! For our sake of measurement, you are not to alter the can in anyway prior to the test. The naturally rounded surface provides the extra bounce to resist standard airsoft fire at 3 feet. When using the soda can as a chrono, hold the can FIRMLY about 3cm - 5cm away from the muzzle of your gun, being careful to place your fingers strategically out of the line of fire. You also want to be careful about ricochet so wear safety glasses and use as few fingers as possible to hold the can so as to reduce the chances of getting hit during testing!
To start the test, begin by estimating which surface is best suited for your gun. If you have a stock or mildly upgraded AEG or gas pistol, use the side. If you have a moderately modified AEG or pistol, use the center part of the bottom. If that proves to be no problem, aim for the side part of the bottom (ie. the edge where the bottom surface meets the sides). The edge portion is a little tougher than the central part of the bottom surface. Now for those really highly modified weapons with maximum upgrades, aim for the top, which is a real challenge that few can meet!
Obviously nothing beats the accuracy of a real chronograph (which we used to measure and translate these "can" measurements). Shooting cans is the most quick and effective way to gauge the approximate power of your weapon. To this day, local airsoft players still denote their guns to be playable even when their guns are "can bottom capable". However, "can top capable" weapons usually bring fear to opponents who often would rather not play with you!
Note that all test are conducted on standard thin skinned soda cans. Make sure not to use thick skinned soda cans as can be found in some countries. For example, some countries in Asia and Europe offer thick skinned soda cans that are as hard to penetrate as an HFC22 canister!
Note that all tests should be conducted from a distance of 1 to 2 inches away from the can surface.
Here are the approximate FPS ratings for your pistol if you manage to penetrate any of these surfaces in ONE shot! By penetrate, we mean that the BB must enter cleanly right through the surface and end up on the other side. Cracks and near breakthroughs don't count!
ONE SIDE: (BB remains in can after penetrating one side) - 290 to 310FPS
BOTH SIDES: (BB makes it straight through and exits on the other side) - 350 to 370FPS
BOTTOM (CENTRAL) - 420 to 450FPS
BOTTOM (EDGE) - 450 to 480FPS
TOP (EDGE) - 480 to 510FPS
TOP (CENTRAL) - 520 to 540FPS
*All figures are approximated based on tests with an 0.2g BB. Varying weight of BB may alter results as the above FPS values only apply for 0.2g BBs. Using 0.25g BBs will sometimes allow you to penetrate a surface even at the lower FPS rate. All FPS measurements chrono'd with a Combro cb-625. The range of FPS above accounts for minor structural differences between cans which can require a little more FPS to make the mark.*