The Salvo rifle was a US military design project from the 1950′s. Essentially, it used two barrels to launch 5.56 caliber projectiles to get as much lead down range as possible.
The rounds were based on the 7.62×51 cartridge that was then necked down to take a lightweight duplex load. So, with each pull of the trigger, four projectiles launched. Click the above link to get the details on the double barrel Salvo rifle.
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