I've experimented with the Hornet for years. It was the very first caliber I ever handloaded for. I've likely tried every trick in the book with this caliber, and on this subject, here's my opinion...
The Hornet, as I'm sure you know, has a very tempermental attitude some times. It'll shoot a load great, then just by increasing, or decreacing the charge weight by 1/10 or 2/10 of a grain, it suddenly gives you horrible groups. There's just not much room in that little case, so every change you make is magnified. A normal caliber like say, a 223 will often only show small changes when you adjust the charges 1/2g, or even a full grain. Try that with a Hornet, and you stand a very real chance of tearing something up, or possibly blowing up the gun.
Back to sizing...
If you're full length sizing Hornet brass, it's so thin that it'll actually cause pressures to run lower. The brass expands to the point that it gives the powder more room to burn, and in turn drops pressures. If you'll try loading a proven, tested load in full length sized brass, then the same load in neck sized brass, then shoot it over your chronograph, you'll easily see the difference. The expanding brass uses enough energy to kind of cusion the gases, making the full length sized load shoot slower. The neck sized brass will run hotter. In such a small, thin case, it can take a perectly safe load, and turn it into an over load real quick.
The best accuracy I've ever gotten from a Hornet comes from partial sizing through a full length die. Set your die about 1/10th inch off the shell holder when you're sizing. This will keep the cases true(neck sizing dies can cause rediculously bad run-out), and it'll make them head space off the shoulder, plus, it'll leave a slight high spot at the base of the neck which centers up the neck in the chamber.
If you'll size them that way, the cases will last longer, and accuracy will usualy be better.
