I own a 338 Ultra can. What is the minimum barrel length I can use on 300 RUM when using the 338 Ultra? I'm currently shooting 212 grain ELD-X bullets, 91.5 grains of retumbo, out of a 26" 1:10 barrel. MV=3180. I'd like to try running a 20", 1:9 barrel with the 338 Ultra can on the end of the rifle. Is the 338 Ultra rated for that configuration? Many thanks!