I saw a video once where a guy took .22 cal pellets (for an air rifle) and used an L shaped allen wrench to push them through the chamber and into the threads on the barrel of his .22. Then he put those .22 blanks behind them and shot them. If i remember right the low power one shot the pellet around 1500 fps and the high power blanks launched it at about 3000 fps. I think he reported that accuracy was pretty poor.
I've thought about trying it myself but never got around to it.