I just purchased some "subsonic" 147g 9mm FP bullets from Missouri Bullets. What are "subsonic" bullets? How are they different from regular bullets? Any advantages? Or is this just a marketing gimmick? Thanks
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Most 9mm loads for 147 grain bullets are subsonic because the case isn't large enough to get enough powder in to launch that size bullet any faster within safe pressures.

So it probably doesn't mean a thing.

LWD

Sub-sonic is ammo that does not break the speed of sound and does not give the hyper-sonic "crack" associated with faster rounds. It is generally 1040 fps or slower, depending on your ammo and the gun you are using it in.

Don't think that subsonic ammo is "silent" as some have come to believe. So don't get rid of your muffs just yet...

Sub-sonic 9mm ammo was developed for law enforcement use in suppressed sub-machine guns. At about the same time, they were looking for pistol ammo that would not "over penetrate" and the 147 grain hollow point fit the bill for both applications.

I thought the question was refering to bullets and not ammunition, in which case LWD's answer would be right, I would think. Those bullets would be made to stabilize in the 9mm at sub-sonic velocities - they are heavy.