1/4" "K" copper tubing, OD = .375", ID = .305", comes in coil, but would straighten pretty easily in short lengths
or
ACR straight copper tubing, refrig & AC, 3/8" od, .030" wall

Either would fit in .375 H&H cases, length would determine seating depth of the bullet, = bullet base stops on mouth end of copper tubing. This would decap with a Lee 22 cal decapping rod, no need to take the tubing out of the case.

With the case mouth expanded, either would fit in 30-06 cases. FL size cases after installing tube.  Seating depth could be set ? with part of a 5.7X28 case turned/filed to .306? set on top of the tubing/soldered to the tubing.  This combination would also allow decapping with Lee 22 decapping rod.

However. If this is about reduced capacity cases, I have pretty convincing data suggesting that reduced capacity cases do not improve accuracy with cast bullets all that much.
I have fired tens of thousands of 30-06 cartridges with 9-12 grains of Unique and very acceptable accuracy.

Same with 300 Win Mag

Or, reducing volume with COW or grex or sawdust or Knurlman powder gives acceptable accuracy.

and
Step-necked cases allow no-sizing and finger-seating bullets to the step/stop.

Any comments from J. Louis or BP or that Kaires guy?

Cat_Whisperer wrote on Jan 6^th, 2014 at 6:19am:

Too small for what?
Why solder, it ain't going anywhere?
It's big enough to take up room, to decap.
It may expand to the walls of the case.
The tubing or the 223 cases work in my 375 H&H cases.
Amy Madigan: "Are we going to talk about it, or do it?"

Good info.  Understand the collapse.
My preference is to machine it after casting it full of tin.
thanks.

I remember seeing that add but, it was quite a while back. 

Yes, what you suggest could be done. I'd make the heads out of SS. I wouldn't cut the case on the head end as that end would give you the most strength and best diameter for threading and then cut it to length.

Frank

Sounds like some reinventing of the wheel is being attempted.

If you've got a copy of Hoyem's The History And Development Of Small Arms Ammunition Volume Four, look on page 36 and you'll find a cartridge called the .40-70 Remington Reloadable, which is described as a .40-70 Sharps case with a screw-on steel head.

And if you go to pages 206-207 of the same book, you'll see a patent application filed on February 4, 1874 by Frank W. Freund, of Denver, Colorado Territory, that looks like it could be an interesting way of making a reduced capacity cartridge (for black powder anyway).

Why Not just machine a pice of Brass to the large case in question....and then machine the inside to the capacity that you would like to work with?
Doesn't seem like it would be too hard to make one?
I'll have to try it sometime. 

Terry

I’m kind of with broken arrow on this one. I also have seen what happens when tubing is used. Many of our newer members weren’t shooting when basic cases weren’t available guys who wanted to shoot say, 45-100 used to take 45-70 heads turn down the heads and solder tubing to them and trim them to length. It is still recommended as the only way to make some shells In Donnelly. I have seen cases split ends separate and in one case it contributed to a rifle coming apart catastrophically. 
  Secondly, if the body is threaded perfectly and the thread is 100% thread and the case is perfectly sized to the chamber then when the round is fired and the case expands, gas will escape through the threaded portion of the case. 
  It just doesn’t seem like a good idea to me. Rocky Mountain Cartridge, while a more expensive option, at least gives a solid case to work with.

40 Rod