Guess it's basically the same question as asking "How much variance in bullet seating depth is acceptable before it causes measurable differences in group size?"
Question that should be asked is how much variation in neck tension and resulting bullet movement affect powder burn efficiency and chamber pressure build to influence maximum average chamber pressures and muzzle velocities. If you are not looking for utmost accuracy, .005" may not matter for shorter distances to target but if you are wanting accuracy at longer ranges, .005" movement of bullet is sign of poor neck tension that should be addressed.
If you are getting COAL change due to chambering, in or out, you do not have enough neck tension. Period. Your ignition reliability is in question, at best.
As to the question itself: yes, 5 thou may be sufficiently enough change to influence your load performance. In some cases, that might mean a destructive, catastrophic over pressure event, or it might be as small as a slight shift in POI or velocity - or might be small enough that your well developed load doesn’t shift appreciably within the expectations of your application.
Good point.
Many bench rest/long range match shooters turn necks to produce consistent brass thickness for more consistent neck tension which in turn produces more consistent pressure build. If you have poor neck tension, you can kiss accuracy goodbye no matter how consistent your powder charges are.
(While you are loading .300 BLK, when loading for pistol calibers, especially small internal case volume 9mm case, neck tension and bullet setback is particularly pertinent as inconsistent neck tension can not only affect accuracy but significant bullet setback can create over pressure situations)
COAL also should not by your critical dimension for almost all bullets. BTO is the critical dimension. Nothing touches the tip except the target.
Another good point.
Bullet length and ogive (bullet nose profile) can vary quite a bit depending on brand and bullet type and when looking at COAL consistency, you are actually looking at distance from start of rifling/lands to ogive and use of comparator is more pertinent.
(For pistol calibers, especially for straight walled semi-auto calibers that headspace on case mouth, distance from start of rifling to bearing surface/bullet base that engages the rifling and bullet seating depth are more pertinent with consideration given to resized case length variation - Shorter length case should be used to set OAL)
And it's good that you are checking for "chambered COAL" as many reloaders focus on "finished COAL" only.
It doesn't matter what your "finished COAL" consistency is to accuracy. What matters for accuracy is "chambered COAL" if you are looking for utmost consistency of your rounds and smallest group size.
(And for semi-auto pistol calibers that bumps the nose of rounds on the feed ramp with availability of bullet diameters that can vary, "chambered COAL" is significant to accuracy and .005" seating depth can matter -
https://www.thehighroad.org/index.p...neck-tension-and-bullet-setback.830072/page-4)
I usually have to load with at least .003 to .004 neck tension in my ARs.
If you are already running at least that much, then could there still be some case lube in the necks?
Good point.
This myth busting thread showed lube type used could affect neck tension and bullet setback (or forward in your case) -
https://www.thehighroad.org/index.p...affect-on-neck-tension-bullet-setback.834035/