There are not guarantees when it comes to custom rifle builds.
Even the best barrel makers can turn out an inconsistent barrel.
Today, most barrels are pretty good, but it's still luck of the draw.
If you're going to get a custom build done, you don't mention chambering to a wildcat or one of the improved versions of a standard off the shelf cartridge.
A custom 30-06 with a standard SAAMI chamber that doesn't rely on custom handloads, will retain more of its value than a 30-06 Ackly Improved chambered rifle.
You also have to consider a lot of other aspects, such as custom trigger, stock, magazines etc
If you're getting a local smith to do the job, as well as work up a load with YOUR preffered bullet shape/type/caliber, expect to pay a significant premium. This would include providing the set of dies, cases, bullets, powder, primers and even some loaded ammunition, with proof targets.
The premium could easily be as high as the cost of the build. TIME IS EXPENSIVE.
I bought a custo Kenny Jarrett rifle appx a decade back. Ordered it directly from Mr Jarrett's website and after some telephone confiremation.
The fellow I spoke to was very knowledgeable and helpful.
The rifle was chambered for the 280 Remington, in a Tri Lock action, bedded in a lovely custom synthetic stock.
This rifle would shoot off the shelf Remington factory loaded, 150 grain Kore Lokt offerings into under one moa, if I did my part well.
The rifle actually shot better with custom handloads, tailored for it.
Jarrett would not guarantee any of their builds, at that time, to consistently shoot ANY factory loads into less than an inch at 100yds.
This isn't surprising at all, considering how much components change from one lot to the next.
Here's something from their site:
Jarrett Barrels
In the 1980’s, during his quest for the ultimate in hunting rifle accuracy, Kenny Jarrett discovered that 27% of the high quality barrels he was buying would not shoot to his standard of sub one minute. These barrels were well built to the bench rest standard of the day, which required a bore diameter with a consistency of 3/10ths of a thousandth from the breech to the muzzle. Kenny determined that to achieve 1/2 M.O.A. performance, a barrel must have a deviation of no more than 1/10th of a thousandth. No one manufactured a barrel with that standard, so Jarrett began its proprietary barrel program. Each barrel receives 1100 hand lapped strokes to achieve the 1/10th of a thousandth deviation, assuring performance that has set a new industry standard.
I sold that rifle, not because it didn't do as promised, but because when loaded to its full potential, the recoil was brutal.
The rifle only weighed 8 pounds (3.6kg) with scope, fully loaded. The 22 inch barrel was good and didn't significantly change point of impact from a cold first shot to the fifth shot.
It was a great rifle. The young fellow I sold it to, from Cranbrook loves it.
Did I recoup my costs? NO.
That rifle didn't do anything better than any of the CNC manufactured offerings off the shelf at the local gunshop, at less than half the price.
