Nobody has replied, so I'll try to help.
I don't have or load for a .30/378, but have the next closest thing, the .300Rem Ultra Mag. Though, I did get to put 12 or so rounds through a Weatherby Stainless SBGM in .30/378 assisting a fellow trying to sight it in. (He'd bought more rifle than he knew what to do with. He burned through almost 2 boxes of ammo and had run out of adjustments on his scope before I saw what he'd done and took pity on him, and saved him some ammo sighting it in for him- another long story for another day- Gun showed back up at shop he bought it from after deer season, with some curious scuff marks and dings.....)
The first thing you're going to encounter is the cost of ammo and brass. Figure around $3.00 per case. Not terrible, but not cheap. Ammo last time I looked is around 90.00 to 110.00 per 20. You might try Grafs.com for the best prices on cases and other components including shipping. They have the best shipping deal going.
Given that you have a few cases, your going to need the correct components.
The Hottest magnum primers are what are called for, meaning either the Federal 215 or the Winchester Large Rifle magnum primers.
Powders are going to be limited to the very slowest available, meaning Retumbo, Reloader-25, H50BMG, and US-869. I'll mention some others that you can mull around on the internet and possibly find some available and that is: WC-860, WC-872, and IMR5010. These latter are GI-surplus powders that are much cheaper. Look at these at
www.gibrass.com. I've done business with Jeff Bartlett and he's as stand-up guy. I bought my stash of WC-860 from Wideners Shooters supply several years ago, for $25/8lbs(several kegs!) so, that tells you how much prices have increased in 5yrs.
You'll also need better than "normal" bullets. Anything but informal target shooting is just going to be too much for them given the velocities of the .30/378. You should also plan on using 180gr or heavier bullets as this will slightly reduce the volume of powder, velocities, and rate of barrel wear. For a serious hunting bullet, the Nosler 180-220 Partitions, 180 and 200gr Accubond, and either the Swift A-frames or Scirocco's.. (I've been "burned" by Hornady's too much to recommend you risk a $$$$ hunt with them. Though you may find the GMX or Barnes Triple-shok-X bullets to your liking. But they are $$$ expensive) Sierra's 220 and 240gr Match Kings as well as the Berger VLD's will be the most accurate "target" type bullets, but I doubt that you'll do much target shooting other than load work up's, so pass on those.
Lastly two additional points:
1. Recoil is going to be substantial unless you have a muzzle brake. If you do have a brake, muzzle blast (noise) is going to be substantial. Use ear plugs AND ear muffs. Especially if you're watching someone else shoot the rifle.
2. With the above powders, the cartridge will not lend itself to significant charge reductions. More than a 10% reduction, and perhaps even as much as any more than 5% reductions can cause dangerous hangfires or misfires resulting in a bore obstruction, which if not corrected can cause a disastrous "KA-BOOM". So, do some research on, and get some reliable data from creditable sources. Try
www.hodgdon.com,
www.alliant.com and
www.hornady.com for starters.
Remember, the useful bore life on such monster as you have is rather short. Figure on around a 1,000rds for match type accuracy and 2,000rds "maybe", for Hunting grade accuracy.
I suggest that you pick a particular load to hunt/shoot, and then stick with it. If you wear out the barrel before the owner makes it back to the states, you both might be rather ill when you find that it costs another $500.00 to replace the barrel, on top of the cost of the components/ammo you've burned through.
I've got around 300rds through my .300RUM and have all my loads "worked out" including some reduced loads, (if you can call .300winmag equivalent loads "reduced"!) But I've enjoyed the rifle, even when it drew blood on "both ends" when I shot a couple of deer with it.
FWIW; I've found that of the formerly "bargain" priced bullets, that the Remington 180gr CorLokts work really well. Most accurate bullets from my gun, and I bought 500 when on sale for $48.00 about 5yrs ago. They also have held together and penetrated well on the deer I shot with them. Another bullet I intend to try is the Prvi Partisan 190gr BTHP from Grafs.com. It should be a good "plinking, target" bullet if sufficiently accurate. (my rifle is just a 1.5moa gun, so, thats just fine with me for my intended purposes; though I have seen better groups-thats the average)
I'll wear out the rifle before I wear out the brass I've got (168pcs), and I'll probably bequeth the rifle before I get around to shooting it another 700rds. I've got too many other rifles and projects planned to spend any more time with the .300RUM.
Too many guns, hunting trips; too little money and time..........
Good luck, and good shooting.