Title said head space gauge.....
But the OP's comment was:
New to reloading and was told by my friend i should get a head space gauge for the cases. Which is why I suggested the Precision Mic.
It seems to me that a firm fitting case would be centered in chamber and cause the bolt to end up being square if there was slop in bolt.
A very popular belief, but another myth in the shooting sports reloading world. The front end of the case will be centered, but the back end won't be. It's the back end not being in the same place for each shot that causes accuracy problems.
Bottleneck cases headspacing on their shoulders center perfectly up front in the chamber where it counts when the firing pin drives their shoulder into the chamber shoulder perfectly centering it when the round fires. Doesn't matter how far the case moves from its resting point to shoulder contact with the chamber. Doesn't matter how much clearance there is around the case behind the shoulder either; centered is centered. And if the bolt's got an in-line plunger style ejector, the round's centered in the chamber up front as it gets pushed forward after the bolt's closed.
If the case headspace is .001" longer than chamber headspace for bottleneck cases headspacing on their shoulder and the bolt's closed on one, some binding happens. If the bolt face is out of square with the chamber axis, previously fired cases in that chamber will also have case heads out of square. If the case isn't indexed in the chamber the same way it was first fired, the high points of both bolt face and case head increase the binding effect and make the bolt harder to completely close. That off-center high-point contact place is where the force axis will be when the round fires. If there's any slop in the bolt fit to the boltway in the receiver, the front of the bolt won't end up at the same place causing the locking lugs to seat at different places against the receiver. All these force points cause accuracy problems as they're not repeatable from shot to shot. This is why the top high power match rifle competitors full length size their fired cases setting their shoulder back only a couple thousandths to ensure their rifle's bolt closes exactly the same for each shot. It makes the difference between a 1.0 and a 0.5 MOA rifle and ammo at 600 yards. The more the rifle's bolt face is out of square, the bigger the problem is. All of which it's important to have enough clearance lengthwise on the case to let the bolt close to exactly the same place for each shot. And if the locking lugs aren't lapped to full contact as the bolt closes to the same place, they'll have different contact areas with the receiver as the bolt head comes to rest at different points and that causes accuracy problems.
Even with severe out of square bolt faces, new cases (with reasonably square heads), excellent accuracy is possible. For example, good lots of commercial .308 Win. match ammo shooting under 2/3 MOA at 600 yards all day long in properly rebuilt M1 and M14 service rifles. Why? everything's pretty much repeatable from shot to shot.
Never read any issues with neck sizing in Precision Shooting or on 6mmBR.com.
Those competition rifles they discuss have their bolt faces squared up and that minimized the issue mentioned above. But top ranked service rifle shooters using M1 and M14 competition rifles know very well that when you load a round in their chamber, you don't bump the op rod handle "to make darned sure it's closed right" at all because doing so will reposition the bolt and op rod and if they both ain't in exactly the same place for each shot, accuracy goes down the drain. And no 'smith I know of (military or civilian) ever squared up those service rifle bolt faces which is why they never shot resized cases fired in them very accurate; case heads were permanently way out of square.