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Understand that the chamber of any centerfire rifle that uses a bottle necked cartridge is cut to dimensions meant for an individual chambering only. the chamber has to provide support for the brass cartridge at all points to keep the brass from bursting when the cartridge is fired.
Also the bore must make contact with the bullet to send it in the right direction and keep the projectile from skipping down the bore and doing damage to it. Or if the bore was too tight it could cause severe overpressure that would actually cause the barrel to burst.
Now if you bought a AR15 chambered in 7.62x39 and bought a 5.56 upper half, then you could shoot 5.56 when that upper was installed, and 7.62x39 when that upper was installed.
Ar-15
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