I bought a .338 WM 40 years ago and love it. Were I buying an elk gun today (knowing what I know now) I would definitely go for the .338 RUM instead! While I don't own a .338 or .300 RUM, I do load for a buddies .300 and I'll admit, I do love the cartridge!
Personally speaking, I think even the 300 WM is a bit lite for a big bull elk unless you're fairly close and are using a good bullet, but I am *very* opinionated about this subject, having seen way too many elk hit and wounded with lesser calibers, from .270 to .308, 30-06 and especially 7mm Mag. Even the 300 WM is not exactly a lightening bolt on elk or moose. While all of these calibers will, when conditions are perfect, kill an elk, however those perfect conditions seldom happen to me! Your luck may run contrary to mine, so use whatever you like. But elk are big, tenacious critters and to anchor them down to one spot, I like my .338 WM in combination with my favorite bullet, the Nosler Partition. For the .338 RUM that would be a 250 gr. while I run the the 210 gr. from my .338 Win Mag. I also limit my shots to 400 or so yards, maybe 500 if conditions are perfect.
I'm assuming that if you're building the rifle you also reload. I've been told that you can't get more than one, maybe 2 loads out of RUM brass but when working up loads for the 300, I got as many as 15 to 20 loads from the mostly Winchester brass I had (brass for either caliber is a huge problem right now as it's hard to find and *very* expensive!). I would bite the bullet and buy as much Lapua brass as I could afford, were I starting out now. Assuming you could even find it, that is. Gonna cost you a bundle but if you want to play, you're gonna have to pay, assuming you reload. I can't offer much in way of info regarding factory ammo as neither I nor my buddy can afford such things!
Once I got familiar with the rifle I started out much higher up the scale of loads, as I wanted the best velocity I could get, with accuracy being a second consideration only after getting the velocity as high as I could (using a 200 gr NP. This makes load testing a bit on the expensive side!). Once max velocity is found, then you can mess with seating depth and perhaps lowering the velocity a bit to tune for accuracy. But it turned out that brass life was pretty good by only sizing the brass enough to get it to chamber easily (this is a Rem. 700, that coincidentally had what I would consider to be a match barrel. It was dumb luck but it's a great shooting rifle!). I used the same brass for accuracy testing as I did for the load work ups, so none of the brass was "babied" with lite loads!
I would also be prepared to install another barrel if the first one decides it doesn't like Partitions. Although I refuse to hunt big game with anything other than the Partition, the downside, besides the cost, is some rifles simply don't like them. I've sold rifles that wouldn't shoot these bullets because while I'm usually fairly broke, I'm also very picky about what I hunt with.
Call me hard headed, you will be right and won't be the first but it's just the way I am and at my age, I'm not likely to change no matter what anyone says! I know what works for me and I'm sticking with what works!
Cheers,
crkckr
P.S. The .338 Lapua is another alternative cartridge to consider. I don't know what the ballistics are but I believe they are similar to the RUM and I've heard good things about it's accuracy. It could come down to brass availability!
