My muzzleloading experience is with snowshoe hares rather than squirrels, but long ago in a state far away I did a load of squirrel hunting with handguns, shotguns and rimfire rifles. Based on using the same calibers now on hares plus a bunch with muzzleloaders, I think it's fair to draw conclusions.
Head smacking with anything more than a 22 rimfire is going to remove lots of head. Even 22's will do it with HPs. Nature of the beast. I'm currently using 32, 36, 50, 54 and 58 caliber for hare heads. All of them will leave a headless critters, even with reduced loads if you get low in the skull or into the neck. Nature of the beast again. Even 20 grain charges in 32 and 36 caliber will take off the far side of a head. As will 22's, as a matter of fact.
I'm just not sure that you're going to start saving heads by reducing caliber unless it's accompanied by a major drop in velocity- certainly more than you would want to consider if 50 yard shots are on your plate.
The only thing I've found that doesn't mangle heads all that much is standard velocity 177 pellets from an air rifle. By standard velocity I mean less than 800 fps compared to the over-1000fps "magnums." In my first hand experience those too remove a lot of head.
With all that in hand, I still think a 30 or 25 caliber would be a worthy project. I just don't want you to get your hopes up about saving more heads. Aint going to happen, but there are still ample reasons to build them for squirrels.