I'd suggest something other than a 7mm Remington Mag. IMO, the performance gain of the R-7 is NOT that substantial unless going beyond book max.
Well that's just it with the Rem Mag or any caliber for that matter. Most handloads will out perform most book load maximums. It is not good practice to try that, but if you do pressure testing, generally you can go 1 to 2 grains over max. Safety first when doing any reloading.
For a pressure test, I will generally load anywhere from 6 to 10rnds starting at a safe load which is usually 2 to 3 grains under max and move up one grain per round. I only load one shell because I am just trying figure out the max. No need for accuracy testing yet. I find it helps to save brass life and bullets. Once I do that I will pick 3 or 4 loads that tested safe and then break it into half grain increments in 3 shot groups. It would look like this;
1 round each for testing. Powder, RL17 caliber .308 Win. Bullet 175 Nosler custom comp
44.0, 45.0, 46.0, 47.0, 48.0= 48 is safe but showing a little ejector marking
3 rounds each 175 Nosler custom comps
46.5, 47.0, 47.5= currently loaded ready to accuracy test.
When I shoot these, then I will go to 5 shot groups for verification of accuracy depending on grouping.
You could do this with a 7 Rem Mag and find it will out perform the book.
Tank