Reloader’s Update:

Hull Tube Separation

Unlike metallic cartridges and brass shotshells, separation of plastic and paper hull tubes from the short brass base is not an uncommon phenomenon. When this happens, inexperienced reloaders sometimes immediately leap to an erroneous conclusion that the load is demonstrating "pressure signs." Experienced shotshell reloaders know that paper and plastic hulls do not exhibit visual signs of pressure. Hull failures and separations are due to either reloading error(s), dilapidated hull condition or a combination thereof.

Prior to reloading a once-fired hull, it must be closely inspected for signs of stress including; fractures in the brass or plastic sections, loose brass and worn out crimps. The sequence involved in the combustion take a toll on hulls. The thin plastic tubes, used on modern day shotshells, is obviously going to show signs of wear more quickly than an all-brass casing. Like anything else on this planet – whether it be animate or inanimate – a hull will fail after an infinite number of uses. The laws of physics furthermore dictate that hulls give out at the peak moment of stress (i.e., during the the combustion sequence). Inevitably, a reloaded tired, old hull will fail regardless of the load it contains.

The most dramatic symptom of hull failure is called hull separation. The firing process continually stretches the hull’s tube and creates weak points along the length of it. The actual separation or disengagement of the tube with the hull’s metal head can be the result of several factors:

1. The head may have been manufactured in such a way to be predisposed to separation; the metal head may have been placed on the hull with less than the necessary amount of friction to prevent the plastic portion from pulling free of the metal base.
   
2. The reloader may have loaded the shell in such a manner so as the crimp will open by force of the wad wall. A shell’s crimp must open up through upward pressure by the shot column -- concentrated from the center. If the reloader allows the shot column (i.e., the height the pellets rise to in a given load) to sag lower than the leading edge of the wad, the wad itself can become crimped in the folds of the hull closure. Subsequently, when the load is fired, force is placed upon the outside edge of the hull plastic by the wad edge (instead of properly opening up the crimp from the center outward). This misplaced force may actually separate the plastic tube from the metal head or, if the hull is softened by the heat of the burning powder, the upward force may separate the tube at its weakest point -- sometimes seen with Activ hulls1. The stronger and thicker the wad in the given load (such as steel hunting wads) and the lower the shot column contained within the wad, the more likely conditions for hull failures and separations are present.
   
3. The reloader must use cork or felt filler wads under the shot (used to raise the shot column height to the proper height) in order to bring the shot column height level or even slightly higher than the leading edge of the wad. Set-back will force the shot column back into the shotcup during transition through the barrel. Remember, changes in shot size used will change the height of the shot column (larger pellets take up more space) even if the same weight of shot is dropped. Final determination of a satisfactory column height is left to the individual handloader.
   
4. In general, lead shot target wads will rarely cause hull separations because the plastic of which target wads are made is very soft and the petals are generally thin. Additionally, target wad payloads are small and the height of the shotcup is usually shorter then the amount of shot dropped.
   
5. Some powders generate more heat during combustion than do other powders.   Generally, slower burn rate powders prolong the hull's exposure to hot propellant gasses. Slow burning powders are almost always used for high velocity / heavy payload shotshells. The combination of excessive heat and stress can greatly contribute to hull wear and tear. The plastic hull may show pock marks, strips, grooves or fractures. Hulls with any of these flaws must be thrown away.
   
6. Dished, or concave crimps are usually signs that something in the reloading process is flawed. While following a recipe to the letter, the reloader must keep in mind that many reloading principles become variables vary within each recipe. In other words, not every load recipe is going to follow the same rules as another. For example, shot size greatly affects the fit of the load. Shot size you use may differ from the load that was originally developed and tested.
   
7. Humidity can cause some elements of the load to swell or shrink. Powders must be continually checked for ratio of weight to volume - with final measurements always calculated with a calibrated grain scale. Even different lot numbers of the same powder will change in volume/weight ratios. It is up to the reloader to assure that all these factors are correct, measured by weight and that all the conditions have been met in order to produce a good and consistent load.

1.  "Weakest point" may be compared to a chain. If any link in the chain is weakened, the entire structure of the chain is at risk.
2.   For further information on this topic, please refer to The Powder Manual.