I was wondering if it was possible to improve the 338-378 weatherby case. I noticed a lot of people are geting just over 3000 fps with the 300 SMK and wonder how much additional velocity could be gained.
To answer your question, yes, a velocity gain can be had and with the addition of long barrels that velocity can get as high as 3360 to 3400 FPS with the 300 gr bullet.
Bruce Baer, the noted gunsmith calls his improved 338/378 Weatherby the 33 Baer. He makes it from the 378 case or the 416 case and when improved they both hold the same amount of powder. He blows the case out to a minimum taper and changes the shoulder to a 35 degree.
Two other fellows besides me at the Williamsport 1000 yard range have the 338/416 Rigby imp and they are getting 3360 FPS from their 44" barrel monster guns.
The one just put a 46 " gun together and he expects 3400 FPS.
The best I have got from mine was 3310 FPS and the barrel is 37" long.
The same thing can be obtained from a 378 case if it is improved exactly like the
416 is and with a 35 degree shoulder instead of the Venturi shoulder.
Check with Bruce Baer for more information on his 33 Baer. His name is always mentioned when talking about 1000 yd match shooting and LR hunting. He has built a few World Record guns over the years.
Sorry it took so long to answer this.