Over recent years I’ve completed a good bit of testing of rimmed straight-wall black powder cartridges that headspace (see note below) on the rim. The testing consisted of: firing primed only cases, primed cases with different non-explosive fillers and a bullet, black powder cartridges with over-primer wads (OPW), BP cartridges with various thicknesses of under-primer wads (UPW) and thousands of cartridges with various powder charges and a variety of bullets. The result is a pretty clear understanding of the progression of events when a cartridge is fired. Notes: - All the testing and shooting and the following observations are based on the use of standard large pistol primers. Using magnum pistol or large rifle primers may have different results. - Headspace: SAAMI defines it as, “The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case rests” - For a discussion on using OPWs and UPWs, see the article at the following link: http://www.texas-mac.com/Large_Pistol_Primers_and_Primer_Wads_in_BPCRs. html
There are several conditions that affect how a cartridge reacts when fired including: chamber/throat dimensions, chamber conditions, how a cartridge is loaded and the components used, to name a few. Generally, the firing pin strikes the fully seated primer and shoves the cartridge forward until the rim makes firm contact with the rim seat. As the primer ignites, the force from the primer alone is more than sufficient to drive it backward out of the primer pocket until it contacts the breechblock resulting in primer setback unless the case stretches back, contacts the breechblock and reseats the primer flush with the rear of the case head. The primer explosion simultaneously forces the rear of the powder column forward and ignites it. As the powder ignites and chamber pressure increases, the case expands and the bullet obturates (expands in diameter and shortens slightly), “locking” the case against the chamber walls prior to moving forward.
If an UPW is used to cover the flash hole, the primer explosion forces the wad forward. The primer flame jets around the wad, igniting the powder. Relatively thin (0.004” or less) paper UPWs will usually be burned up by the primer flame and/or burning powder. Thicker UPWs will be scorched but remain intact and either remain in the case, be sucked into the bore or completely out the barrel by the pressure differential as the bullet moves forward. A primer will generally not “punch through” a UPW thicker than 0.004” or so. Remember, these observations are based on using relatively mild large pistol primers, but I would expect similar results from "hotter" primers. If an OPW is used, the wad is seated and supported by the bottom of the primer pocket. Due to primer pocket limitations the OPW is typically no more than 0.008” thick when used with large pistol primers in rifle cartridges. Since the OPW cannot move forward the primer explosion will pierce the wad igniting the powder.
Next, the case expands as the powder column ignites and chamber pressure rapidly increases. Assuming the case was full-length resized, it will expand several thousands and, depending on the case size; expect it to shorten 0.005” to 0.010”. If the case was fire formed and not resized, typical expansion will be around 0.001” and shortening will be negligible. As the case expands and the bullet obturates and starts to move forward along with the unburned powder and over powder wad, the case will grab the chamber wall assuming the chamber is not excessively wet or oily. Although the case locks against the chamber wall the pressure is sufficient to stretch it back enough to reseat the primer flush with the rear of the case. By the way, if you doubt the case is stretching back to reseat the primer, fire an unloaded case with only a primer. If the primer remains slightly backed out of the primer pocket but is flush after being fired in a full powder load, it’s a clear indication the case is stretching back slightly, but it’s minimal and no reason for concern assuming the headspace is not excessive.
If the chamber is coated with oil or excess moisture from cleaning, wiping or from using an improperly designed blow tube, the now forward-moving bullet may pull the neck portion of the case forward while the rest of the case slides back, potentially resulting in a stretched, split or completely separated case. Other factors that increase the possibility of stretching, especially with dirty or rough inside case walls, are: relatively long cases, heavy powder column compression which will increase powder-to-case-wall friction, over-powder wad-to-case-wall friction, using LDPE or HDPE (poly) over-powder wads, bullet-to-case-neck friction resulting from heavy neck tension, bullet crimping and necks that are too soft due to excessive annealing. For an in-depth discussion on case stretching see the article at the following link: http://www.texas-mac. com/Case_Stretching_and_Separating_in_BPC_Rifles.html
If the chamber pressure is insufficient to fully obturate the bullet and tightly force the case neck into the chamber wall, a dirty outside case neck will result due to burnt powder residue being blown back between the case and chamber. The most common method to correct this condition is to raise the chamber pressure by increasing the powder charge. Another approach that may work is to delay bullet movement using one or a combination of the following: seat the bullet so that the nose is “hard into the leade” when chambered, crimp the bullet, and/or increase the case-to-bullet neck tension. But be mindful that, as discussed earlier, excessive crimping and neck tension can result in case stretching.
Once the bullet moves forward, the chamber pressure drops and the case retracts (pulls away) from the chamber wall slightly (diameter decreases by 0.001” to 0.002” at most), allowing for easy extraction.