My only concern with something like this if used indoors is that the gases from the lead styphnate used in primers. From what I've read about it this is a far bigger source of lead passing into our systems than handling lead bullets. Without the ventilation system used in a properly set up indoor range I'd say this is a non starter idea. Far better to stick with a common air or CO2 pistol.
Indoor primer shooting isn't new either. The pellet trick is a good one but there have been other systems in the past. I recently bought a box load of reloading supplies and it came with the Speer Target 38 plastic bases and plastic "wadcutter" bullets. The plastic bullets in this case are reuseable provided an energy absorbant target or backstop is used and like the Convert-A-Pell system it uses rifle primers as the power source. But instead of the barrel insert and brass holders it uses just the plastic cases and bullets sized to suit the gun.
