Sorry to disappoint some of you, but Federal GM hulls are billed as one piece, because the separate base wad is "bonded" to the straight walled hull. When you cut them apart you can plainly see the seam. Some of them are not so well "bonded" and fall out when cut apart. If you put them in the freezer for a while after you have sectioned them, you can knock most of the base wads off with just a tap.
I have had instances (2 or 3) where the base wads separated from the hull after firing. They were still in the hull proper, but closer to the front as throw some kind of suction pulled them forward. I will say that those hulls had been reloaded 3 times. I've also had my ejectors rip the whole back section of the hull off, but leave the tube section stuck in the chamber. I'm going to opine this is gun dependent. Although it happens to others, I have a friend who says they last forever, especially the old ribbed ones. Go figure.
Phil, IMO the best hulls for reloading are Remington STS hulls (green and gold). Gun clubs reload well also, but do not last as long. STS hulls are truly one piece, as you can easily demonstrate by cutting some apart. They have an smaller interior volume than other hulls, and the cuvrature of the base is substantial. For that reason, Remington wads work the best. The pattern superbly also, so no worries there. The DRM replacements fit well also, but I have not patterned them yet. Wads designed for straight walled hulls, or even the Federal wads will cause some fit problems in an STS hull.