The sizing die is the real issue for the fellow wanting to do a 25-06 Ackley improved chamber. But you have more than one option:
* you can order in a custom set of dies from Redding, Forster, or Lee, and be at their mercy for delivery.
* you can send thre to five once fired cases to Linwood Harrell, or Neil Jones (quite possible you best bet)
* you could also buy a Newlon die blank, and have your gunsmith chamber it, but you'll then need to have it hardened. I'm not all that sure about the hardening process for 12L*** series steels, so I won't go there. He also sells a stainless steel blank. Or you can cut a die from a piece of 4150 pretreat steel and simply nitride harden it (this is your best bet in my opinion). You'll see very little shrinkage, and if done right the finish will simply need a fine polish. The nitride process is very simple, and very hard.
The seater is easy. Just buy a 25-06 Forster seater, and run the chamber reamer thru it. Or better yet use the .257AI seater die spool ( they will sell you individual die parts). That's a lot cheaper than buy a generic seater in a semi custom die set.
Now for the three to five once fired cases, that part is also easy. If the chamber is cut right, all you need to do is to run some factory loads thru the rifle. Use round with heavy bullets (like 117 grain), or load your own. If you do the load your own process, try jamming the bullets into the lands a couple thousandths. I like others here, like to fire form with good stiff loads of fairly fast burning powder that develops a good amount of chamber pressure. Try 48 grains of AA4350, or even 49 grains. Their 4350 is slightly faster than the other 4350's, and will develop about 62K psi. Chambering a new 25-06 case, you should feel a slight resistance at the very bottom the bolt closing stroke.
This assures you that the case body is locked in place solidly. The bullet into the rifling just increases chamber pressure, and further helps with alignment of the loaded round in the chamber. I would expect about .035" of case shrinkage after forming. But it might be as high as .045" because your moving a lot of metal in there.
The 25-06 improved is one of the easier rounds to form. Try something like a 30 Meyer!
gary