Attached is a picture that shows another way of making longer brass. In this case for a Soper rifle for 450 x Two and three-quarter. As you can see in the picture, I took a 4570 case turned the rim and drilled and tapped it for 5/16, 24. Then turned up the extension piece and screw it on. Also, help you reduce the load. lol.

Thanks Rodney! 
Now my head is spinning thinking about guns I passed up or sold because they might hurt me from the huge case capacity! That's a great idea!

Puts me in mind of the O'Connor "Steelheads" from the 1980's. The concept was to have brass case bodies with screw-on stainless steel case heads. They came in .30-06 basic straight and (I believe) H&H Magnum basic straight, wherein one formed them to whatever cartridge case one wished within those two cartridge families. Touted as being intended to allow a lot higher pressures without the case heads and primer pockets stretching.

They worked, but weren't cheap. I bought a set of them in a plastic case, a half-dozen maybe. Sized/formed in a .30-06 die, loaded and shot them, then found that the steel heads were inexorably jammed onto the threads of the brass bodies. (Male threads on the perimeter of the brass, female threads in the steel, If I remember correctly.) I reckoned it was too much of a good thing and moved on. Might still have them buried somewhere.

Years later I heard rumblings that perhaps they really were too much of a good thing, that wild-eyed daredevils took them as insurance to be able to load at crazy high velocities with attendant crazy high pressures. They overlooked that brass is intended to fail before the gun fails and some idiots got hurt. I bet that (plus their high price) caused them to fade away before they really took hold.

I love Rodney's approach though since it's apples and oranges to what I described. Who in their right mind would jack pressures up to 70-80K psi in an old straight walled cartridge single shot anyway? 

What brass alloy did you use, Rodney, if I may ask?

Good idea Rodney!  So does this mean you've had some range time with the Soper?

The machining brass is also called 360 Brass. It most often comes in a 'half hard' annealed state.

Rodney, 
Do you drill a center hole in the plug to allow powder to fall in? Just curious if you need to do any trick to ensure the primer provides a good flame to light the powder. 

-Dave