The cases stretch stretch during sizeing much more then when fired. If you measure a new case, load and fire it, then remeasure, it will likely be notably shorter. This is very apparent with straight walled pistol cases such as the 45 acp. The brass forms to the chamber diamentions when fired and as the walls expand outward the brass not only thins but draws from its lenght. The brass then "springs" back to allow the case to be extracted from the chamber, but it doesn`t return fully to its original geometery. The sizeing die then compresses the case walls back down dureing sizeing and the displaced brass needs a place to go, the case "grows" as the brass flows foreward. Brass does not thicken back up, it stays at the thinner size it was stretched to when fired. Think of a balloon, squeeze a blown up one and the balloon will grow longer but the walls remain the same. The "sloppier" your chamber the more the case will expand outward and show "growth" when resized. The semi autos with their larger chambers will allow more brass flow then a tight chambered bolt or single shot with a snug chamber. Cartridge doesn`t matter much in this process, the case to chamber fit is the major component. Although I have heard the more taper there is to a case the more stretch they will exibit. I can`t vouch for that though.
Neck sizeing or partial sizeing reduces the amount of brass displaced and allows for more fireings then one will get from full sized brass. Neck sized cases will go 10-12 cycles in some of my rifles without trimming and end up being tossed just because I feel they are nearing the end of their life without anealling, which I don`t play with. The cases used in this manor will show neck cracks from work hardened brass before the case body shows any sign of seperation.
The lenght of the case neck IMO doesn`t mean much as long as it holds the bullet firmly. A few thousanths under trim to lenght won`t hurt and Ken Waters once wrote of trimming .005" shorter then the books figure simply to reduce his time spent trimming cases for a rifle that known to cause excessive stretching.
The purpose of trimming is to keep the end of the neck from entering the throat of the barrel and being "pinched" causeing the case to grip the bullet too tightly. This will raise pressures in a otherwise safe load. The fact the bullet "jumps" this little bit of neck area is mostly negated IMO by the fact the bullet ogive is already past this point and already in the leade near the lands of the barrel. There is not much the oversize area of the chamber neck can do to cause the bullet to veer from its path straight in the bore. A bench shooter will of course with not want this condition, but he already has a chamber that is as close to perfect a fit to his cartridge as possible. The person with a rifle that shoots 0.5" or larger groups likely will never notice any differance in accuracy from a case that fills the necks lenght totally vs one that is 0.010" short.
In the end the only way to reduce trimming is to reduce the amount of brass you are displacing. this can be done with "tight" chamber fit or by causing the cases to fit the chambers tighter useing a neck sizer in place of full sizeing. Heavy loads will also cause the cases o form tighter to the chamber wall and not spring back as much as lower charges to so degree. This can cause the shoulder of the case to cause tight chambering and require you to "bump" them back occasionally. This is usually done with a full lenght sizer although there are "body" dies that do this without touching the case walls. Cases will normally grow some when this is done.
How often or how much brass we remove at a time doesn`t affect case life. You can remove it 0.001" at a time or 0.010", in the end you will take the same amount off by the time the case needs to be replaced. I prefer to take large amounts at a time and only trim 4-5 Xs dureing the cases life. if one wants to trim daily he can, but I see no use in it in most cases.
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