I can make brass from the 338 Lapua if you need some.
I use WC820 mil surplus powder and H110 as well With either one start around 23 grains and work up from there depending on barrel length you shouldn't have to go over 27 grains to transition to supersonic then back off if you don't have a chrono
You might also consider LilGun. I like it better than 110 and have found to be cleaner. Depending on barrel length starting 23gr would be a good start.
I am going out today to shoot a revolver match but, after I am going to try out this rifle. I still have to break it in but, I will try to get about 5 rounds down range.
18" douglas barrel 25.8 grains of WC820 CCI standard primers 690 AP projos produced 1034 fps avg. using my Oehler 35p with a 3/4 MOA group with 2 in the same hole. not bad for military surplus. powder and projos