Like I said earlier, the height of the "brass" varies, as does the hull color & crimp, but those things don't matter for reloading data. It's still the same straight-walled interior with the same Hi-Power paper basewad, so the internal components don't care. That's why those Federal hulls are all lumped together in reloading data.
Here are some pictures. Please excuse the complete hash I made of sectioning these; I'm moving soon, and all my good tools are boxed up in a storage unit. I had to cobble together two rusty old hacksaws to get a working one, and the only blade I could find looked like a hobo's smile.
This is an older Hi-Power-style Top Gun hull (one of the pink breast cancer awareness ones) compared to the guide in Lyman 5th Edition:

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This image shows four sectioned Federal hulls. The middle two are older-style Top Gun/Field & Target hulls. The one on the left is a fired hull from a vintage (mid-80s, probably) Hi-Power Duck & Pheasant load that was hanging around in my dad's old shooting stuff. These three are all Hi-Power-derived hulls with the same internal construction & geometry. The hull on the right is one of the new plastic-basewad Top Guns from the 100-shell value packs sold at Wal-Mart. You can see that in addition to the change in material, the plastic basewad has a different shape & depth than the paper Hi-Power basewads.

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I've read some theories online that these new Top Guns are based on some Euro hull, and can use that data, but I never heard a definitive answer for which one exactly is a good match. I never really bothered to spend much time looking into it since I almost never bother with these cheapos - old or new. But if you're not as much of a snob about hulls as I am, maybe you could do some digging and see if someone on another forum has figured it out.
Keep in mind, I'm not saying
don't use the newer Top Guns if you come across them. Just be aware that Federal doesn't recommend them for reloading (or at least they didn't, when I contacted them about it a year or so ago), and that they are a different hull design that can't be counted on to produce the same results with data meant for Hi-Power shells. Maybe pressures will be lower, maybe they'll be higher, maybe the difference will be negligible, maybe the answer is all of the above depending on the wad or propellant you're using; I don't know. Just be aware that shotshell pressures are hard to predict, and that there's increased risk when you stray from published & pressure-tested data.