ndbwhunter, By cost I mean if you have a standard mag action then get the 338 RUM or 338-300 ultra barrel from Pac-Nor, screw it in and your good to go. Much cheaper than doing the Lapua unless you happen to have a wby mk 5 action or sako designed for the Lapua. Back in the 90's when I worked quite a bit with 338's off the Lapua and ultramag case the 338-300 ultra was the go to cartridge if a guy had a standard mag action and wanted a big 338 because he could do it cheaply. That was the beauty of the 338-300 ultramag. But if you are spending the money for a custom rifle
the 338-300 ultra would not be a good choice because there are so many better choices if you want to spend custom dollars. In 2001 the 338 ultramag came out filling the same niche as the 338-300 ultra so not much need for that one anymore although I realize the fad of owning one seems to keep it alive for some reason. Just to many better choices out there in a custom. In the late 90's I spent a lot of time developing a great 338-300 ultra and built near a hundred of them back then. The introduction of the 338 RUM filled the same niche in an inexpensive rifle and with better choices in a custom me and most others dropped the 338-300 ultra in 2001.
With all the work I did back then shooting all the big 338's if you averaged all the data the lapua was probably 50 fps faster but so negligible I say they are equal along with the 338 RUM. If you did 10 of each there would not be 100 fps difference in the average accuracy load velocities between the three in same length barrels. If 100 fps makes a difference to a guy then get a big gun like one off the 378 wby or full length 416 case and go well beyond these three.
I have been there, done that and got the ball cap, tee shirt and coffee mug. Few people have the experience with specializing in the big 338's over the past 35 years as I have. I was there testing way before the 338's got to be the thing to have.
For some reason the ultramag case is extremely accurate in 30, 8mm.338, 358 and 375 calibers. It is just a natural and that maintains the popularity of the cartridge. But the Lapua is just as accurate and so are some others.