I think the most overlooked measurement in handloading is neck clearance. This is simply the amount of clearance a loaded cartridge has around its neck when nestled inside a chamber. But how much clearance? Fine guns need precision ammunition, after all.
A lot of people with more knowledge than I suggest the average rifle should have a chamber neck which is 0.002–0.003 inches in diameter greater than the ammunition being used in it. This clearance is necessary for reliable feeding and for the brass to expand into, so that the bullet can be released uniformly.
Insufficient clearance can cause dangerous pressure spikes and will surely ruin accuracy. It's a situation that can arise when repeated loadings cause cases to stretch and necks to grow thicker, making this a measurement worth taking.
A quick and dirty way to check neck clearance is to grab some cases that have been fired in your rifle and inspect them to make sure the case mouths aren't bent or dented. Take a half-dozen cases with perfectly round mouths and try to insert a bullet into the case mouth by hand. If it slides in easily, your case neck clearance is likely fine. If force is needed to get the bullet into the case mouth, or if it won't go in at all, there could be a problem and you'd best follow through with the next step.
Take at least three of those cases, full-length size them, and then use them to make up dummy rounds containing a bullet but no primer or powder. Measuring the neck diameter of these dummy cartridges is easy: A caliper will give us that number quickly.
Ideally, we can then compare the average cartridge measurement to the neck diameter of the chamber. However, measuring a chamber's neck area is difficult. A chamber cast is the best way to do this, but that's overly complicated for most handloaders. All we want to do is ensure there's sufficient clearance in the neck area for safety purposes and maximum accuracy, so a simple go/no-go gauge will suffice.
It's easy to fabricate these using your dummy cartridges and some thin transparent tape. Place one wrap of thin tape around the neck area of your dummies being careful not to overlap the ends. A small gap is fine. Now measure the neck diameter again and if you've added 2-3 thousandths to the measurement, you're good.
Now carefully slide these dummy rounds into your rifle's chamber, seeing if gravity is sufficient to drop them completely in. If you need to add some finger pressure, or use the bolt to chamber the cartridge completely, your neck clearance is likely insufficient and it's time to take action.
This means investing in neck-turning tooling and reducing your case necks just enough to get that 2-3 thousandths of clearance. Turning case necks is easy and we'll look at that another time.
But, guess what? Just like most handloading processes, it requires precision measuring. There's no way to escape it.