the .17hmr is a .22 magnum case necked down to the .17 diameter. It is the same as a 177 pellet I think. As best I can remember it took a while to make a.17 in hard steel barrels due to the fact that the tooling to cut rifling had to be developed to be strong enough that small. Pellet rifles are softer metal, some even brass, like the Sheridan's
Steve