The following post contains "on the fly" math.
The slow video function of WMM is not enough without a high speed camera. All the program does is take the number of frames your camera recorded and increases the display time of each. So, when you reduce 30fps to 1/8th time you get 3.75fps, 240fps goes to 30fps at 1/8th speed. Now, factor in pellet flight time and distance to target.
Acording to my Strelok app a pellet shot at 900fps will take 0.3 sec to reach a 100yard target. That equals 1.125 frames recorded at 30fps and 9 frames at 240fps. This ratio is why you need to record at high frame rates to be able to see the flight on video.
If any of this is incorrect, someone please educate me.
EDIT: I got a few minutes free to check my math. I got the flight time numbers wrong. See post below for correction.