The Savage custom shop is more of an erector set operation and are fine people to work with.
They take existing components and assemble them in ways not found in the catalog.
For example, Savage had made 7 x 57 barrels before but never offered a left-hand model. The left-hand model needs no modifications to accept a 7 x 57 barrel. So easy assembly. You got a custom gun.
They have also made me a left-hand 375 H&H with two complete bolt assemblies -- one push-feed and one controlled-feed. For those interested, neither has ever produced a failure to feed, extract, or eject.
They have made me a left-hand single-shot rifle in 25-06. Did not machine in the magazine hole and did machine in the typical single-shot action cartridge slot.
They do not do things that are totally from scratch that is not in their factory in any form. Mostly they rewrite factory routers and make production lots of one instead of fifty.
However, Remington, Winchester, and Colt don't run true custom shops either.
I think of the Remington custom shop as upgrading wood and a little engraving. Ask them to make a left-hand Remington 1100 in 16 gauge and see what they say.
Years ago I asked Winchester to make a custom Model 70. Described exactly what I wanted verbally, in writing, in drawings, and in person. Their basic position was it was impossible at any price. No interest at all. The thing was Winchester had built the exact rifle that I was describing in the 1950s, I knew its entire history, and had it in my possession (where all the measurements came from). I didn't tell them the last part. Winchester is no longer the rifle company it once was.
At a national show I spent time with the Colt custom shop guru asking for a specific SAA revolver -- again one they once made. He assured me that it would be no problem. Two weeks later back at his shop he sadly informed me that they couldn't do it. He suggested that I go to USFA -- who built it with no problem.
I've even had custom Rugers made back when Bill Atkins was alive.
I have had many custom guns built. I expect to pay the price.
But the mainstream gun companies are no longer in that business.