Probably not very. 3 feet at most.
I did see a video of a guy taking a .22 cal pellet and shooting it using a nail gun blank in a regular .22 rifle. The FPS was pretty impressive, somewhere around 1,800 fps.
I think I saw that video too - velocity was much higher than that. Those guys were drilling laser-like holes in metal targets with those pellets.
I think with a bolt-action .22, a tool made from a chamber flag to consistently seat the pellets at the same depth in the breech, and maybe a heavier pellet design - that could be a very effective combo when "normal" ammo is scarce.