I have found that with necked cases, there is some case length growth as the cartridge is fired but more so most growth happens as the case is sized. The sizing die presses the case neck and shoulder back down from its expanded size (after being fired) to bring the neck dimension down to a slightly smaller diameter than called for. Then, as you draw the case out of the sizing die the ball on the decapping pin is pulled thru the neck resulting in the case ending up with the proper ID at the neck to properly retain the bullet. Different manufacturer's dies may vary this ID size by +/- 0.001". This drawing of the ball thru the neck tends to pull on the brass case making it longer. You can measure the difference by measuring the overall case length both before sizing and after sizing w/ a caliper. I find the length change varies per case.
Factors such as case lube, brushing the inside of the neck, the brand of case, how work hardened the brass is, the smooth shape of the pin's ball... can all play a factor in the amount the case is drawn out and lengthened. Polishing the ball will make it both smoother and smaller. Both reduces how much the ball pulls on the case mouth. But you don't want to reduce the ball OD much or you will increase how firmly the case will "grasp" bullets pressed in. Light polishing is fine.
No matter how much you try, after a few reloadings the case will have lengthened so much it will now be outside the case OAL specs. The case length for .223 Rem = 1.760". The advertised trim length = 1.750". As long as your cases are in between these measurements you are safe and within specs. Can you run cases shorter or longer than this range of case lengths... yes you can. Will it be a problem... depends on your gun and chamber. By remaining within the cartridge specifications you ensure the cartridge will work in ANY gun chambered for the cartridge. You will run into a problem when your case is so long that its mouth makes contact with the narrower freebore area of the chamber and the case becomes compressed when the bolt closes.
This pic I came across shows the dimensions of different chambers based on .223 Rem / 5.56mm NATO / .223 Wylde.
View attachment 1024297
Note dimension "K" which is the case length. For the "PTG 223 Rem Match" you would never want the case to be over 1.760" as it would be too long. For all the rest, case lengths of 1.770" should still chamber fine. When the case is so long that the mouth makes contact at point "F" you have a problem.
For me, I measure all my cases after resizing but before the primer is installed. This means resizing and priming as two separate steps. I do like priming cases by hand.
By checking their length after resizing, I can confirm if the resizing step has stretched them. Any cases I find over 1.760" go into a bucket to trim. Anything less than or equal to 1.760" goes into the reload bucket. I trim the cases down to 1.750" before returning them into the assembly line.
When I am trying to make a batch of very accurate loads I will trim all the cases in the batch to the same length (1.750") just for consistency sake. This is just another way to minimize the tolerances so each round is consistent and the overall batch produces the smallest grouping.