shotgun hulls whats the diff

Okay, I bought a shot shell loader about a year ago its a nice one, pacific. and I got a book and followed the directions for the wad, shot, and charge all works well. I checked here (THR) when i was ready to load slug rounds, got the mold and poured them, loaded as per the book. Test fired them at the range and they cycled just fine in my 1100 tactical. All is good but for the life of me I can not understand or see for that matter what the differance is between hulls. The size, length, every thing looks the same to me. Brass okay low brass high brass but the slight differances do not seem the big.

So to my question, why do the loading manuals spec. a certain hull for a certain load. Unlike rifle or pistol you can use any brass as long as the caliber is correct you are good to go. X amount of powder and X amount of shot with a wad is going to take up X amount of space so why this change for winchester AA, but this wad and this shot with remington peters hulls even though the amount and size of the shot is the same? I dont get it!

It's all about internal volume. Less volume with the same powder charge will raise pressures. Some cases have base wads, some one piece with no fiber base wad, some tapered walls, some straight walled. All differences will change pressure and a maximum load in one case could be over maximum pressure in another. Lymans Shotshell Handbook has some data on pressures and its effects when changing hulls, or primers.

I must admit I did stray from the manual........but only once. I used an old paper hull that was in my hull drawer (always knew I would reload shot shells) I think it was a federal. Reloaded it to winchester spec. it shot just fine.

IIRC, shotguns operate at about 10-14k psi. In one test, just changing brands of primers changed the pressure by 7k. At 14k max, you have a much smaller margin of error than you do for most pistol, and certainly for most any modern rifle.

Take a pocket knife and section a shotshell right down the middle to the brass (steel) head. When you do this you will see that Winchester AA hulls and Remington plastic hulls (Game loads,Gun Club, STS) will get thicker as you get closer to the brass head and are taper walled. These are compression formed or one piece hulls and have the least interior volume. Federal(paper and plastic) Estate and many European hulls are straight walled with separate base wads. This style has the most interior volume. If the same load is loaded in both styles of cases the pressure and velocity will be higher in the taper wall hulls. If you take a load listed as max for say a Federal hull and load it in a Winchester AA hull you will most likely be over max safe pressure. Shotshell recipes are component specific and if components are changed pressures and velocities can sometimes change dramatically.

Years ago when I was loading a lot of shotshells, I would cut or section a case and record load data for it every time a got a bunch of something different.
The board is still hanging over my loading bench.

Note the huge difference in internal volume in just this small sample.

http://i81.photobucket.com/albums/j219/rcmodel/ShotshellCases.jpg

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