May I ask why the effort to keep rifle velocity subsonic?
The point of a suppressor is to quiet the muzzle blast, it can do nothing for the sonic boom the bullet makes. Shooting unsuppressed that "boom" is masked by, or blended with, the muzzle blast and not as discernible. To make the shot even quieter shooting suppressed, the bullet velocity needs to be kept under the speed of sound. That's kinda the mechanics of it.
The reason people want to quiet their shooting varies and I can only speak for myself. I like to shoot, but I'm not fortunate to live in a secluded area and I have neighbors a few hundred feet on either side and a mile behind. I'm not a fan of packing up equipment and driving 5-15 miles to one of the local ranges, unpacking, paying a fee and dealing with the inevitable know-it-all idiots and the "range nazi". I do have a safe shooting spot out back with a large dirt berm, but it does nothing to abate the noise. I have good neighbors and respect their right to the same peace and quiet I enjoy, so I limit my shooting of "noisy" firearms to only a few rounds a year. After I got my first rimfire suppressor, I discovered the joy of just walking out back with my .22 handguns and running hundreds of rounds through them, without hearing protection, and not disturbing the neighborhood. After finally receiving my Hybrid 46, I'm hoping to be able to replicate that to a degree with some of my centerfire rifles.
My reason, YMMV
