I have a 338 Khan (388-378 improved) with a 31" barrel and h50 is my powder of choice after testing h1000, retumo, r22, r25, ect. I got the best vel/ pressure ratio lowest ES and its a very stable propellant! 113.3gr of H50BMG under a 300gr vld gets me 3110 fps and zero pressure signs. Its a SLOW burn rate so if a long barrel is there, id make it my first choice for a 338 wether it be a rum or edge or any big capacity magnum. Make sure you use F215 only tho!!! Big volume loads need lots of fire to properly ignite.