Yeah, the 200 grainer is light for the 338, but you had said the 338 shoots heavier bullets faster than the 300 win does, so I was just trying to agree with you. I picked a weight that was heavier than the 30 cal's 180gr, and to get the equal velocity it had to be 200 grains. Anything more in weight, and the velocities are slower than that of the 300 win. No one is here is trying to handicap anything. The 338 is a fine cartridge. Personally, I've always thought the best weight for the 338 is 225 gr at about 2800 fps, but hey, that's another discussion.
As for putting a magnum... really ANY magnum, in a barrel less than 24", well that's just crazy. We could go the other way around, though. What if we put the .308win in a 24" tube? would it step up in velocity like the 300 win does? It will increase a bit, but not as rapidly as does the 300 win. Actually, I also think that a 30-06 with less than 22" isn't at it's best either. So yeah, if you've got a 20" bbl, you might as well shoot a .308. Otherwise you're not seeing the benefit of the larger case capacity.
Most reloading manuals use a 24" bbl for the .308, and still list the top velocities for 180gr bullets at about 2500. The manuals will also have a 26" bbl for the 300 mag. and list the top velocities at 3000 fps for a 180 gr bullet. So that's only 2" difference. If we bring the .300 mag back to 24" to compare with the .308, then the 300 will lose about 75 fps. That difference is still more than the 300 fps difference you were talking about. Now, if we take the .308 to a 20" bbl, then it will lose about 120 fps off what the book says. So the 180gr bullet is really only going about 2380 fps.
So, a short bbl for a .308 [20"], compared to a short bbl for a 300 mag [24"] will give a difference of 545 fps. That is a big difference. So let's make the .308 a 22" bbl., since that is also common. You'll be at around 2440 fps. Compare to the 300 mag with a 24" tube [short by magnum standards] and the difference is 485 fps.
Okay, now lets take the 300 mag to 22" [ridiculous]. It's still at around 2850 fps. That's still 400 fps faster than a 308 with the same length bbl.
Never has the difference been 300 fps.
That's what I was disagreeing with. And with normal bbl lengths the differences are 500fps or more. Period.