Massage gun listings lead with speed counts because they are easy to compare. Speed is the least useful of the three numbers that matter.
Amplitude
Amplitude is how far the head travels on each stroke, usually 10–16 mm. It determines how deep the percussion feels far more than the speed setting does. Not every maker publishes it; when one does, it is worth noting.
Speed range
Expressed as percussions per minute, often 1,800–3,200. More settings is not better than the right settings. A gun with five well-spaced steps is more usable than one with thirty that all feel similar.
Noise
Quoted in decibels, typically in the 40–60 dB range. A gun at the top of that range is the one that stays in the cupboard. If you plan to use it in a shared room, this number matters more than the speed count.