Coke can test is unreliable and I'll tell you why.
Different batches and different bottling companies can have slightly different thicknesses or other properties in the can leading to slightly differing structural properties. Don't get me wrong it's an alright test but should never be used to get anything more than a precursory glance as to what kind of speed the gun is shooting at. I'd say at best you're looking at a potential variation of up to +/- 20 FPS give or take.
Instead it might be a better idea to do the "microphone test" which I am a big proponent of. Basically setup 2 microphones a known distance apart and use a recording program to record the "woosh" (at it's peak) of the BB as it passes by the mic.
Using grade 10/11 physics formulae you can then solve for speed knowing distance between mics and the time the BB passes them. (v=d/t and assuming you set distance in metres then you get metres per second (obv), then to convert from m/s to fps it's velocity*3.281).
Example: Say you setup your mics at a known distance of 5 m apart and it takes 0.05 seconds for the BB to pass by both mics as measured by your computer/recording program you can then solve it to be shooting 100 m/s or approximately 328 FPS. This way is a lot more accurate than the coke can test, although not as good as a "real" chronograph test.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE