I've put about 2200 rounds through my 338LM. About 150 of which were 250 MK's. The rest were Lapua Scenars and 300 MK's. I had very bad results with the 250 MK's even in the factory 1 in 12 twist barrel. 250 grain Lapua Scenars out performed them by magnitudes. I don't have the number in front of me, but the bc for the Scenars is significantly higher than the MK's. I cannot prove that the Scenars shot better than the 300 MK's in the factory barrel, but I liked them better. It was too close to call. I've had numerous 6" groups at 1000 yds. with the Scenars.
I have a new Rock barrel with a 1 in 10 twist, and it definitely seems to prefer the 300 MK's.
Personally, I think 250 MK's suck and I will never buy another one.