ok, went and read the previous posts. sounds like you have pretty much exhausted the possibilities. i agree with most of what was said and the guys on this post are pretty knowledgable, great resources. i think that it is a simple problem, but i doubt we can get to the cause without seeing the gun. i think the cheapest way out would be to get your buddy's sendero and shoot that one. when you want to go to 338 you can just rebarrel (would blue print the action first).
If you shoot long distance, i would definitely go with the heaviiest best BC bullet out there. the drop is well worth the difference in wind bucking capabilities. with your 300 try 150, 180, 200 grainers at the600 yard jugs in windy weather. it will astound you the difference in just the 180-200 grainers. then try a 210 VLD or 220 SMK. it is much more significant than the books tell you.
good luck, god bless.