i have 2 of Darrel's 3-12x42 one on a .280AI the other on a .300WM they are excellent scopes i haven't had parallax issues and have used them on steel out to 1000yds with good results. for a light weight rig i don't think you could get a better scope.
kenny jarrett calls it the ".284 jarrett" this is what he has to say about it on his site.
"We were amazed that the .300 Winchester Magnum had never been reduced to 7 mm successfully. The wheels really began to turn wondering exactly what the volume of the case would be. We designed a...
i had a pac-nor barrel in 300wm installed on a m700. it behaved in the same way with lateral stringing i tried everything(different scope/rings/bases etc), took it back to the smith twice in the end i traded it in as it never shot better than 1.25moa . got a Kreiger barrel now no problems.
You would be beter served IMO with 160gn bullets. Ibex are tough animals to put down, I used 180gn ballistic tips in my 300WM with good results on Krgystan ibex last year. If they are out there you normally have time to range them and dial the dope required. the extra bullet weight and and...
If your worried about crap getting in the action and locking it up just get a Mauser 98.
If you want a custom action and the accuracy that goes with it you are just going to have to take a little more care of it.
For a custom hunting rifle I really like the borden alpine and timberline actions.