Edge, don't get me wrong when I say this but since you are talking about a custom LR rifle I would assume that whoever had the rifle build would of selected a barrel twist,
chamber,taken magazine length into consideration on throating, may of send along acouple empty case with bullets he wanted to use and most important had a few conversation with the gunsmith about this rifle and if not an odd caliber what loads did some of his past customers have good luck with. I would think the owner of this rifle would of done alittle homework on what he had build.
I picked up my new elk rig 300Wby yesterday from the gunsmith, headspace was set on the shoulders not alot of freebore like most Wby longer Wyatt magazine box etc. Gunsmith gave me a new powder and some bullets that would work besides what he knew I wanted to use. I ordered a set of shoulder bumping dies from Neil Jones for the 300Wby and some different size busing so I can play with neck tension,seating depth these are arbor dies so I can load at the range for my testing. I'll do some testing during barrel break in as not to waste the barrel. I select powder based on load density and that works best for me before I got the new rifle I picked up 100 cases and played around with some powders that I want to try and looked at load density to base of bullet etc so again alot of homework done before rifle is shot.
Guy walks into a well known BR gunsmith and ask can he build a rifle to shoot in the .0's gunsmith ask can you shoot in the .0's. When you get to looking at LR groups and what defines those groups how much is rifle or shooter. I would assume if you have to ask so many questions and if this doesn't work let's go to plan B if it doesn't shoot better than 1" after you have a custom rifle
I guess we can all assume something.