I’ve never fired/handled guns but have some knowledge of guns and their basic mechanisms, and this thought just came across. If you could make cartridges out of polymer/plastic, and they are significantly lighter, wouldn’t you want to switch to them asap? yet there is no adoption. On the other hand shotgun shells have always been plastic but it’s not problematic in any way? why is this the case?
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The bullet of a rifle create a very tight pressure fit against the barrel. The shotgun pellets do not fit into the barrel in the same way and any wadding is also a relatively lose fit. So the pressure inside a rifle tends to be at least five times higher then inside a shotgun. At the rear of the chamber it is actually the brass casing which seals the pressure inside. The brass expands to make a very tight fit to the chamber walls and the bolt face preventing any gasses from escaping. Shotgun shells do also have brass on the base to do this. But the relatively short brass base of the shotgun shell is able to contain the lower pressures of the shotgun compared to the pressures inside a rifle.
This does not mean that polymer cartridges does not work in rifles. They are in commercial production. However they have to be thicker leaving less room for propellant. And the tolerances are tighter. So to get the reliability to an acceptable standard there needs to be more quality control to find out which combination of ammunition works in which guns and to check that the ammunition maintain quality through their production and in storage. This is therefore mostly done by militaries who go through lots of ammunition and use identical guns. For private users this is very hard to do and results in ammunition which might have been tested in a different gun with slightly different chamber size, feed ramps and chamber pressure being unreliable in this other gun.
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