i have one. it will shoot a 300 at 2800 if i push it, which i don't like to do. i have a rock, 5r, 29" barrel. Roy has a 338rum that shoots a 300 gr well up in the 2700's with a factory 26" tube.the difference between the 2 is not as much as most think. it will take a lot more than 6 gr of powder to make 150fps difference.
I have built a bunch of 338-300 ultramags and have not seen one that will get 2900 fps out of the 300 matchking without ruining the brass. Doesn't mean it can't happen, just don't get one expecting that velocity because that would be above average. I have a 300 wby with a 26" barrel that shoots the 180's at 3430 fps and the 200's at 3250 fps. Those are the accuracy loads. Straight 300 wby. I have a 257 STW and a 338-378 that are also way outside the scale. Sometimes you just hit a barrel that just goes crazy I guess.
A 30 year old man near here, blew part off his face of this year when the bolt blew out and hit him in the face.. If you are ruining brass then you are exceeding the safety factor....
I believe that Shawn stated 2800 FPS with the 300 SMK in the 338 Edge not 2900 FPS.....