Based on your screen name I'm guessing you have a 300WSM. Load it with some of the better 200-215 gr bullets and it'll do anything the 338 will do with 200-225 gr bullets.
The 338 will look better on paper at the muzzle, but you can kill an elk at close range with a 30-30. Beyond 200 yards or so the better BC's of the 30's will translate into more speed and energy than a 338.
I can see an advantage if you move up to 250-275 gr bullets, but will you ever hunt anything where that is needed.
Just to set the record straight, let's compare one of the "better" bullets for the .300WM vs the .338WM. Since I have a Nosler Reloading Guide #7 in front of me and since it has loads for both the .300WSM and .338WM and since it uses 24" barrels for both, I'll use it as a reference. Further, I'll use a MPBR (Maximum Point Blank Range) calculated for a target 6" in diameter.
.300WSM, 200g AccuBond, B.C. .588, 2944fps
Zero Range/MPBR = 254/299 yards
0 yds = +1.5", 2944fps, 3843fpe
200yds = +2.2", 2641fps, 3097fpe
500yds = -31.2", 2222fps, 2192fpe
.338WM, 225g AccuBond, B.C. .550, 2882fps
Zero Range/MPBR = 248/292 yards
0 yds = +1.5", 2882fps, 4149fpe
200yds = +2.1", 2563fps, 3281fpe
500yds = -34.2", 2124fps, 2254fpe
In this case the .300WSM starts out about 62fps faster but at a 300fpe disadvantage. Trajectories are about the same and the .300WSM energy doesn't overtake the .338WM until past the 700 yard mark. Trajectories are very close, within 3" at 500 yards, and the difference in velocity is only 98fps at that range.
Let's just say the comment that " Beyond 200 yards or so the better BC's of the 30's will translate into more speed and energy than a 338" was a little too optimistic in the energy department.
In my experience the Nosler velocity data is also overly optimistic compared to what I get in my rifles. In any case I prefer Hodgdon data because it lists pressure as well as velocity. So let's look at the same AccuBond bullets using current Hodgdon velocity data for bullets of the same weights (albeit different bullets).
.300WSM, 200g AccuBond, B.C. .588, 2827fps. IMR 4350, 63,700PSI
Zero Range/MPBR = 245/289 yards
0 yds = +1.5", 2827fps, 3459 fpe
200yds = +2.0", 2531fps, 2845fpe
500yds = -35.4", 2123fps, 2001fpe
.338WM, 225g AccuBond, B.C. .550, 2832fps, IMR 4350, 52,000CUP
Zero Range/MPBR = 244/287 yards
0 yds = +1.5", 2832fps, 4007fpe
200yds = +2.0", 2516fps, 3163fpe
500yds = -36.1", 2082fps, 2166fpe
In this case the .338WM actually starts out faster with an energy advantage of 498fpe. At 200 yards the .300WSM has just barely overtaken the .338WM in terms of velocity but still lags in energy by 318fpe. At 500 yards the .300WSM still lags in the energy department by 165fpe. There is less than an inch difference in trajectory out to 500 yards and the difference in velocity at that range is only 41fps - not much there to get excited about.
So again it would seem the statement about the.300WSM catching up in the energy department beyond 200 yards was over optimistic rather than fact-based.
The real question is whether or not the differences in retained velocity, energy, momentum or trajectory give either cartridge a significant advantage. While I've found my .338WM to be potent elk medicine, elk haven't always dropped faster than those I've shot with my 7mm RM, .30-06 or .300WM. The difference for me is I trust the heavier .338" bullets to penetrate further and be more destructive when heavy bone is hit. I also like the bigger holes the .338 makes. That said, the 6x5 bull I took this year was at 411 yard and went 4 steps and down after I shot it with my hunting buddy's 7mm RM and a 160g Grand Slam. Placement trumps cartridge.
