UtahConcealedCarry.Com

Thought this was interesting:
https://www.gunsamerica.com/blog/utm-civilian-target-ammunition-the-indoor-training-alternative/?utm_source=email&utm_medium=20160120_SHOT16_2&utm_campaign=/blog/utm-civilian-target-ammunition-the-indoor-training-alternative/

Synopsys, for those not in a position (or not inclined) to open the link:

Ultimate Training Munitions, UTM, has introduced a brand-new line of products designed specifically for the personal defense market with their Civilian Target Ammunition training system. A low-velocity, hearing-safe, non-destructive alternative to conventional ammo, Civilian Target Ammunition is designed to be fired safely inside the home using your personal defense handgun, rifle or shotgun, to keep your shooting skills honed between range visits.

John—this would go nicely with your basement laser setup

Geoff.

Thanks for thinking of me, Geoff. I know, you just want me to be the guinea pig.

I think I'll still with my Smith & Wesson M&P full-size CO2 air pistol for low velocity practice. That said, I may have to try a box just as an experiment.

Bear in mind there's a cost involved to gear-up your gun to use the special ammo.

Pricing varies from kit to kit in the $199 to $539 range

UTM offers a wide range of kits for popular service weapons including pistols and rifle from major manufacturers such as Beretta, Colt, Heckler & Koch, Glock, SIG Sauer and Smith & Wesson. The kits come with everything necessary to get started and only require a field-strip to install. Depending on the model it may come with a complete upper assembly or it may just require a barrel and spring change.

It would be interesting to know how they feel (to shoot—not to be shot by! (just pre-empting Mr. Snurdlebaum there )) compared to an air pistol, or even an AirSoft (not sure exactly what the difference is between those two, other than the projectile).
They say the UTM plastic projectile is only 1g, which is just over 15gr (or less than half an average .22 LR), however the average air gun pellet I think is around 7gr or so for .177 (though average .22 air pellets are more in the same ball park as the UTM at around 14 or 15gr). Velocity is another matter—the article mentions 375 fps but it's not clear if that's the rifle or pistol version. I believe that powerful air rifles get into the quadruple digits in the smaller calibers but I don't know about C02 pistols.
[John—is your S&W an air pistol or AirSoft, what do you shoot out of it and do you know the velocity?]
The UTM ammo is designed to not penetrate skin. I doubt you'd have much trouble penetrating skin with a .177 lead pellet—even from a pistol.

I'm also curious about the expelled gasses too, and the need for extracting them from an indoor environment (primer only but there still must be some, I'm guessing).

So—Yes, John—I think you need to be the Guinea Pig

Geoff.