I have a 24" Douglas 8 twist... I got the 8 so I could shoot the 107 grain Matchkings... Hornady says the 9 twist will shoot their 105 AMAX's in most rifles... so with the 105's you might be able to use the Retumbo load I developed for my rifle...
My rifle is a Remington 788, and the barrel is the 24" light target as I recall... it's not heavy heavy, but it's heavier than a sporter barrel... all else I did to this rifle was I bedded the action and floated the barrel... no receiver work was done at all.
as the target says, 48.5 grains Retumbo with 105 AMAX's... use magnum primers, they do make an ES improvement.
With the 107 SMK's, this is a 600 yard 3 shot group (don't wear your barrel out proving groups... if the first 3 land tight, try 3 more at another distance... it'll all prove out soon enough. The 25 shot aggregate mantra must be being propagated by barrel makers).
The Douglas barrel is a good choice, and as good as anything out there if you ask me... better than most, actually. The glitterati sometimes put Douglas down, not "fi-fi the poodle" enough for them I guess. But they work, and work well...
And with 4831, a slower but still accurate 105 AMAX recipe... see target for load info.
I get 3000 (just over) fps with the Retumbo loads, and just under 2900 fps with the 4831 load.
Good luck with the new build...
Dan
