Hi Theo

Brass is funny stuff mate. If someone heated you to ridiculous heats and bunged you in a pressure cooker, you would get a bit bent.

Cases are quite clever, being harder at the head end, and softer (due to annealing) at the neck end. The whole lot has to be capable of expanding slightly to seal gasses in the chamber, then return to the original dimensions (as near as dam it) so the case can be extracted.

This is handy, firstly as it keeps at that hot high pressure gas away from our mugs, and secondly....well, if you have ever seen the fulton commandos trying to remove a stuck case from your pride and joy it is something you will strive to avoid....aledgedly

Brass hardens with being worked and becomes stiffer and more brittle, so the upshot is that unless we can re anneal (and even when we can) cases change or wear out.

This shows with primer pockets expanding so they can no longer hold a primer in place or seal tha gasses in, as necks splitting, or as groups going west as the neck tensions all go to pot.

You may also get case head seperation, but this can also be down to how well your rifle is head spaced as well as continual firing.

As was pointed out, the brass quite literaly gets forced against the walls of the chamber, under great pressure and heat. It cannot go outwards or backwards, so the only place to go is to creep forward towards the throat of the bore.

The problem is that if you dont keep this within limits, on chambering, you might inadvertantly crimp the round into the case, or increase neck tension to a point where the peak pressures are too high......this can only happen for so long before something goes wrong.

So we keep the things trimmed (this was something I forgot to mention, I tend to trim all my .308 brass back to 2.002" when new, which means it takes real talent to crimp one during a season, and uniforms everything to start with, a top shot passed this on to me and it works...he really hates trimming).

There are 2 main consideration when re loading (being brief).

Safety

Avoiding situations like over pressure etc, things we all do like inspecting cases, load development by safe working up, etc

Accuracy

Things we need to do to give us the accuracy/precision we require.

The latter is where advice can be confusing. Many folks are loath to change what is a winning way, regardless of how logical a change may seem, so their way is the only way and god help anyone that disagrees

The point is that how you need to re-load depends on what accuracy you require, and what is required to get the best out of you rifle.

My brass dont move much on one rifle, so i dont need to work it much (and would strive to avoid that as best I can), but my other 2 are ostensively target or battle rifles, so I dont worry massively as long as they perform as they should.

Only time and some (safe) experimentation will tell you what works best...whatever you do keep a reloaders log and ideally keep test targets (or scans of them) so you can re-trace your steps later down the line.

You may miss a better mousetrap

The best way is to get amongst other folk who are reloading you chambering an dsee what they are doing, it could save you a few quid and a lot of frustration.