Resizing cartridge cases will work harden the brass and increase the chance of it eventually splitting or separating, but there are other good reasons for minimal resizing or not resizing at all. Following is my viewpoint on the subject. While reading further, keep in mind the discussion is for black powder cartridges for single-shot rifles, but most of the comments will also apply to smokeless ammo as well.
As with just about every subject there are pros and cons to consider. If the ammo is used for hunting it’s a good idea to full length resize for adequate bullet neck tension and to ensure the cartridge easily slides into and out of the chamber which may be slightly fouled from previous shots. If the ammo is being fired in more than one firearm, full length resizing is also recommended to ensure the cartridge will fit chambers with slight dimensional variances.
Although long straight-wall cartridge cases can stretch, especially those using compressed powder, if the firearm uses a bottleneck cartridge, due to the shoulder the case is more likely than a straight wall case to stretch when fired. For additional discussions on the subject see Case Stretching & Separating in Black Powder Cartridge Rifles. If the case does stretch, full length resizing will not only reduce the diameter of the main case body and neck but also set the shoulder back slightly. The result is that some of the brass will be forced to the neck by the resizing die, lengthening the neck and the overall case. Sooner or later the case neck will need to be trimmed. Therefore, each time the case is used and full-length resized, brass migrates from the main body to the neck, thinning and weakening the main body of the case, eventually resulting in case separation, typically in front of the thick region forward of the rim. This is better understood by high velocity smokeless shooters but also applies to black powder cartridge shooters, only to a lesser degree.
It’s always been my viewpoint that, for maximum accuracy, the dimensions of the loaded cartridge should match the chamber and throat dimensions as close as possible, resulting in minimal bullet expansion and distortion and maximum alignment with the chamber and bore. Therefore, if the brass is being used in only one firearm, the best practice is to never full-length resize after initial firing, commonly referred to as using “fire-formed” brass. But depending on several factors including dimensions of the bullet, chamber neck, throat and bullet seating depth, minimal neck resizing may be necessary to hold the bullet under slight tension and proper alignment. For those shooters, and I count myself as one of them, that “slip-fit” the bullet by hand with very little or no neck tension, some neck-only resizing may still be necessary for bullet alignment.
If done properly, annealing (heating the brass to soften it) can reverse work hardening of the case neck, but for safety reasons is definitely not recommended for the main case body. For more details on annealing, see to the article titled Annealing BPCR Case Necks.