Winchester makes the cases for Black Hills and the brass is good, thin walls, high tensil strenth with excellent capacity. No reason you can't stuff 45 grains of Varget in them with no drama. I've had several rifles that 45 grains of Varget and Sierra 175 MK's was the bomb loaded COL 2.800.
I prefer RE15 because it runs though powder measures more accuratly and run 44.7 grains under 175 SMK in Win brass, F210M primers at COL 2.800. I get 2725 fps for 10 shot samples over my chony and it shoots into 1 3/4" at my AR10 SPR's 300 yard zero.
Loaded progressive on a Dillon 550.
In your case I would take a look at case prep first.
He's going to need a Stoney Creek case comparator set so he can measure case length from the case shoulder. He's got to make sure he has the shoulder bumbed back around .002 to .003 with a gas gun or it will stick cases and of course full legnth resize.
I would also dump the crimping, unless it's with a "factory" taper crimp die and then make sure it isn't heavy handed. Using a standard seating/crimp die heavy handed will distort the case and cause a raised ring that will hamper feeding in a gas gun and, or stick cases in the chamber.
I've never found the need to crimp for gas guns. I take the expanding ball out of my sizing/decaping die, shaft and all, chuck it in a drill press and take it down in diamater some (.001) with some emery paper, then polish it bright. This gives me plenty of neck tension to keep bullets from moving around when they feed from the magazine.
I have loaded for many gas guns AR10, AR15 and M1A's for both NRA Service Rifle comp and tactical matches. You can invite alot of problems trying to load them like benchrest rifles. Probably the most useful tool you can use is a run-out gauge to make sure the rounds are loaded straight, if they ain't straight to begin with, they won't get any straighter when they make that magazine jump. If groups are important an electronic scale so you can group your loaded rounds by weight. This is double true if you throw powder charges. I don't bother for tactical matches since it has nothing to do with grouping.
Something else I've found is that most of these 50,000 +, getting up there .308 loads have much better ES and SD's when I use magnum primers.
The DI gas guns can be accurate as hell don't make your life more complicated than it needs to be with dubious loading practices