In the rifled guns, the cartridge case of the cartridge is closed at the apex by the bullet, which engages the neck and is stopped in the seat by means of crimping. The case of shot cartridges causes the fractionation of the charge in a column of small independent projectiles, it requires instead of being clogged at the apex by a sealing disc and a rim of stationary, or by a folding towards the center, of the same tube of cartridge case.
The first shutter system is called “round crimp“, instead the second, the most modern, is called “star crimp.” The two systems make cartridge autonomous and transportable because they maintain firmly the structure of the internal components, protecting them from moisture of the environment. The crimp is practiced through a coil, placed in rapid rotation by a manual operation hemmer machine or by an electric motor with a low number of revolutions.
The profile of the coil varies according to the shape of the rim, the speed of rotation and the type of material that forms the tube of used casings (cardboard or plastic). The edging round with section with two folding edges rounded suits lively powders because, of equal height, has a lower resistance than the other types, while the hemming square section, where there are the corners, was abandoned because generates pressures too high and is more difficult to perform.
In casings cardboard is practiced on a round edge on 5-7 mm of the tube, adjusting this height with respect to the resistance of the cardboard, to the weight of the shot charge, the type of wad and the liveliness of the powder. In the cartridge plastic cases the rim height ranges 6-8 mm because this material has a lower resistance at the performance being more malleable at high temperatures of firing. Too low crimps are not recommended because, offering little resistance, cause a slow and incomplete combustion of the powder with the formation of dangerous “fires long.” Regarding the star crimp, is decidedly more modern compared round crimp.
I was produced by practicing at the top of the tube 6-8 longitudinal folds, using a special coil called “engraver” (6 folds is the traditional European close, 8 folds is typically American). Following a second coil crushes these folds towards the center, in contact with the column of the shot, practicing by rotating a circular outer edge, similar to the round crimp. The height of the outer circular edge adjusts the resistance to the carrying out of the closure on the carton and on the plastic, even if the two materials require a different form of engraver and of coil crimp. A regular star crimp closure must occupy approximately 11-12 mm in free cartridge case; this measure varies according to the thickness and type of material of the tube, the height of the outer circular edge, the shape and the width of the edge section. A lightweight closure can be used to reduce the maximum pressure of the cartridge, while a very tall and narrow closure may partially compensate for a lack of clogging.
The round crimp gives an increased resistance in the phase of initial sbossolamento, but an effort of extracting total minor in comparison of star crimp. With the edge round the charge at the beginning of his movement should exert more pressure to curl the tube, then finds a minimum sliding friction within the cartridge case. With the star crimp, the charge exerts less pressure in the conduct of external light edging, while meets a higher resistance and more continuous stretching the folds of the ripple of the tube, during the passage of the shot column and of the wad.
For this reason the star crimp requires a lower dose of powder. The advantage of the star crimp in hunting cartridges is the abolition of the sealing disc, which has eliminated, in part, the famous “empty pattern”; it tends to reduce the dispersion of 5%, because it gets, in the swarm of patter, the “melted “ disposition, facilitating the penetration of the air and thus improving the remaining speed of the lead. In the past, the star crimp gave certain loading problems, employing cartridge cases of poor length, voluminous powder and heavy charges, while today the problem is solved with the use of a cup obturator that allow to reduce to the minimum the height of the column of the wad.