I never shot a 338-408 that is a straight forward neck down. I have heard reports of several different guys who did it, 3200fps was about the top loads. My main loads are around 145grns or WC872 with the 300mk bullet, right at 3400fps. This is what works good in my personal rifle, HBMG-50 groups just as good but with a little more pressure signs. I put on the 35 deg shoulder to get the burning of the powder more efficient. I wanted to keep the shoulder under 40deg, these large cases can be finicky when it comes to feeding out of a magazine. I removed some of the body taper, but the extraction is still easy, even on stiff loads. Loads i tested through my Pro Chono were up to 3520fps with the 300mk bullet. The brass could not take these loads, primer pockets were loose. I need to shoot some through a better chrono and see where i am at for sure.
With the pressure signs i was getting im figuring the actual velocity was much higher than the chrono reported. When i can get to the big range i will run some through the Oelher.
I shot some 250grn MK bullets through it as well, 3750fps on average, groups were very good too. Maybe too hard on the bbl. A 200grn bullet will fly at 4000fps, great 100yd groups, one holers, not much for long range though. A 220 Swift on steriods!