As I understand with shotguns and rifles the regulated components/pressure bearing parts are generally the barrel and action as well as the bolt if the gun has one.
I assume when it comes to revolvers it's the barrel and cylinder.

What is the deal with semi-auto pistols with polymer frames, such as Glocks?

Is it just the barrel and slide that would be considered the firearm, possibly the rails inside the frame?

My reason for asking is wanting to play with printing parts for my Umarex Glock 17 such as a custom frame, without getting an unfriendly knock at the door.

I've already done some fully legal accessories, a flared mag well, faux compensator and a charging handle.