The maximum SAAMI specification for the case-head diameter of the .17 Hornady Hornet and .22 Hornet are exactly the same, .299 inch. This doesn't mean a custom chamber for the .17 Ackley Hornet might not be a little tight for Hornady brass, so it would be a good idea to check the fit of Hornady brass before buying a bunch. But if Hornady brass doesn't, it isn't because the specifications are different.
