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Sunday, October 16, 2011

Romance of the plastic shell

Plastic shell:By the mid-1950s, the wound-paper shotgun case had reached the peak of its development. The folded crimp had replaced the roll-crimped shell with over-shot wad. Winchester-Western had introduced a highly efficient, bottle cap shaped, obturating over-powder wad and their Mark-V plastic shot wrapper that reduced shot deformation and improved patterns considerably. Yet, the problem was in the paper.

Whether the paper hull was waxed or lacquered, it still tended to absorb moisture. I can recall shells that could hardly be seated into the chamber of my gun and others, that upon firing, could hardly be extracted, particularly when shooting a pump or semi-auto. The wound-paper tube had to go, and the major manufacturers like Winchester-Western and Remington consequently spent millions of dollars developing plastic substitutes.

There were endless failures during early attempts to form cases using the injection-molded process. Working with the available plastics of the day, companies found that the resulting cases did not have the necessary tensile strength plus they became soft when heated and brittle when cooled. It took a German technique, the Reifenhauser process, to really bring plastic shotshells to the marketplace by the mid-1960s.

What the Reifenhauser process did was to work the plastic in such a way that its tensile and radial strength were increased during the manufacturing process. In the simplest terms, melted polyethylene resin was extruded through a die to produce a heavy walled tube that was then stretched to six times its normal length, resulting in the resin's tensile strength increasing from 4,500 psi to about 18,000 psi.

The thin walled tube was then expanded in diameter through the use of air pressure, thereby increasing its radial strength. In industry terms, the end product was known as bi-axial oriented tubing. Cut in shotshell length segments, the tube and a separate base wad were then crimped securely into a metal head, producing a straight walled plastic case that was very strong indeed.

Related prouduct: silicone case

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