try rl 22 with a mag primer for 110-120 grain bullets. I recently switched mine from 117 sst to 110 eldx I tried retumbo and could not fit enough in the case to get the velocity I could with rl 22 or h1000. all 3 shot well but retumbo was almost 200 feet slower than rl 22.
try cfe223 by far the best power I have tried in a 308. a lot more velocity than varget with the same or slightly better accuracy. and its ball powder so it's easier to load
just got 2 boxes of the 200 gr from midway called hornady and of course they have no data for 300 rum ill post some when I fig it out and get a chance to see how they shoot.
I can change them that much. just depends on how much the harmonics are changed. taking a pound or close to that off the end of the barrel I can see a 5 inch change in poi.
the adjustable brakes I've seen changed poi very little or not at all but I have only messed with 2 of them.
yes some of them dad put one on my 300 weatherby when I was a kid. id say it kicked a little harder than a 243 but not much more. but like everything else not all of them work the same.
They are great off of a bench or on a varmit rifle. like others have said the Holland is a very good and effective brake. If you plan to hunt with the rifle either have it set up so you can remove the brake or hunt with ear muffs.
i used 180 gr hornady sst's to kill my elk. my brother used 165 sst's to kill his. both bullets were complete pass through's. either weight will work fine for elk.
you dont need those big heavy bullets. that being said any quality bullet out of a 300 rum that you put in the right place will do...