Re: .338 WM Barrel Length
I cut a 338WM down to 22" for same reason. Worked great and didn't lose as much velocity as I expected.
I think with right powder yours would run fine. I wouldn't even be afraid to go with a 20". I used 65.5gr of N150 with 225 Accubond, WIN brass, BR-2 primer and OAL, base to tip, was 3.410 to use mag well. Averaged 2700 fps, with low muzzle pressure and super clean burning.
PM me if you need some short barrel data to get you rolling. Just be sure and let me know which bullet your gonna shoot.