You could try the poor mans chrono (NOT the "Coke can" one)
Setup two microphones a known distance away (5 meters or 15-16 feet) and get a recording program (audacity) to record as the BB passes these two microphones.
You have v = d/t distance is known as you have the 2 mics at a known distance and time is known as well by taking the difference between the "sound peaks" where the BB rushes past the mic. Then you just divide and get velocity either m/s or fps, then if needed convert your answer to something meaningful (ie. joules, or fps or whatever metric is used to test whether or not the gun is shooting hot).
Obviously this can have a great deal of error but at the same time it's a much better metric to test than the "coke can chrono" since coke cans actually have different thicknesses of aluminum between batches (especially between different regions) and can really be inaccurate whereas this is a simple test and only places for variability are mostly human error.
ಠ_ಠLess QQ more Pew Pew
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