Interesting things posted in this thread. Here's my take on 'em.
First off, 63 grains of IMR4350 under a 150-grain bullet in a .300 WSM
case seems to be about 6 or 7 grains below max in several sites giving loads for it. A 10% reduction in charge weight below max often doesn't produce enough peak pressure to push the back end of the case against the bolt face after it's driven hard into the shoulder by the firing pin. That'll end up with the primer sticking out of the case head the amount of head clearance between the case head and bolt face.
Second, if Hart says the chamber headspace is .004" over or long, that probably means it's that much greater than the SAAMI spec of 1.726" for the GO gauge. As the spec for the NO GO gauge is 1.736: that rifle's headspace is right in the middle of the normal and safe range.
Third, no centerfire rifle shooter winning matches and setting records wants any bolt binding whatsoever when the round's chamberd. Even the slightest amount, especially on actions without the bolt face squared and locking lugs lapped to full contact, will cause the bolt to close at different places. That ends up causing inconsistant forces transferred to the barreled action for each shot making the barrel whip differently and the bullets don't all leave in the same direction. I know the "a slight bind" of the bolt on a chambered round is often believed to more firmly and accurately position the round in the chamber, but it's another myth in the shooting sports. If you shoot your stuff well enough to see the difference, you'll agree with me on this.
Fourth and final.... best accuracy and case life typically happens with fired rimless bottleneck cases that are full length sized and their shoulder set back no more than a couple thousandths. You'll need a case headspace gauge (RCBS Precision Mic or equal) to measure your fired and sized cases to see how much setback the shoulder gets. Full length bushing dies from Redding or RCBS are those in vouge these days. Or just lap the neck out of a standard full length sizing die to about 2 thousandths smaller than a loaded round's neck diameter like folks did decades ago before bushing dies were available.