it would be hard to do because the rings wouldnt seat ad there is also no way to crank the engine over because the first crank has to been half of your over all compression then you crank it 4 times and the last reading is your compressionand you cant do that with an air gun cause you wont bespinning the crank at what it should spin at and you have to only remove one plug at a time for pressure reasons at thats why you cant do it out of the car. Now if you want to you can do a cylinder leakage test where you put the piston at tdc an put a certin amount of pressure on it and see how much leaks down, tha is acually a step above a compression test.