You are refferring to the 338-300 ultramag and there are plenty of people on here shooting that round by whatever name so load data is no problem. I built many of these from the late 90's until about 2001 when the 338 ultramag came out with virtually the same performance in a standard cartridge elimininating the need for the 338-300 ultramag. Both are going to hit best accuracy around 3250-3300 fps with the 225's and 3100-3150 fps with the 250 grain bullets. It is going to vary a bit between individual rifles. Both are easy to load for and very accurate. When you can get everything over the counter for the 338 ultramag to me that is by far the best choice and why me and most others quit fooling with the 338-300 ultramag when the standard one came out I think around 2001 or so. Several gunsmiths called them by various names in the 90's and after the turn of the century but most went away with the 338 ultramag introduction. Edge seems to work on this website. I just always stick to calling a cartridge what it is so there is never any confusion as to what it is.
The 338-300 is still a great round but no better than the 338 ultramag. I actually prefer the standard 338 ultramag because it feeds better through standard magnum actions. The beauty of the 338-300 in the first place back in the 90's was a guy could get a little more performance than the 340 wby on a standard magnum bolt face. For the guys wanting a big time high performance 338 and the money for a custom then the big time cartridges are out there overwhelming the performance of the 338-300 or standard 338 ultramag. But the ultramag is still tops on a standard magnum action and I don't see anything out there better than the ultramags for that purpose. They have a niche filled there and do it well.
If you have a standard magnum donor action and wanting to save money the ultramags are a good choice. If you are doing a custom rifle
with custom action you should look at some with performance well beyond the ultramags. Wind estimation is everything in long range shooting and the most unkown variable so all the velocity you can get is critical to iliminate the most of that unknown variable. The high performance 338's are just as accurate as the ultramags so why limit yourself if you are doing a custom anyway.
I have 338-300 ultramags and a 338 ultramag and they are top shooters but they are what they are and no more. Best on a standard mag bolt face. But they can not compete velocity wise with cartridges based off the 378 wby, 416 rigby or improved lapua cases. But for those you have got to spend the money for a wby mk 5 action or a custom action costing more money. Velocity and wind are important. There is quite a bit of difference between the 30-06 and a 300 wby. Just ask anybody who has shot both of those. About the same difference between the ultramags and an improved 338-378 wby. Whole different league. If you are going custom I would consider stepping up so you are not dissapointed if you happen to shoot side by side one day with a guy shooting a big rifle thinking you had the best.
I don't mean to offend anyone on here but I seem to read a lot about the same particular rifles and I want new guys on here to understand a little about the history of all these rifles and what the true performance level is and how they stack up against one another. I have or have had and shot most all of them and have done it for many years. I just want the new young guys on here to understand there are many choices out there and the world doesn't revolve around a couple current popular choices. Talk to a lot of older smiths who have extensive knowledge of big guns and seen many fads come and go. I have no biases and don't care what a guy shoots but I do want a guy to base his decisions with the most information he can gather. I hunt with all calibers based off all the popular cartridges and enjoy all of them because they all have there purpose.