double-barrel buck shot
"Double-barrel buck shot" is actually mixing 2 related terms. "Double-barrel" is any gun using two barrels; they can be "over-under" or side-by-side. Here Beck is referring to a shotgun, because he follows that by saying "buck shot" which, btw, is generally written "buckshot" (one word). Shotgun shells are generally filled with small pellets that spread out after they leave the barrel. Buckshot is larger, heavy pellets used for home defense or hunting large game - like a "buck," which is a male deer. Hence the name.
To punish Beck for inventing the term double-barrel buck shot, I grabbed my grandfather's double barrel from over the fireplace and unloaded two rounds of buckshot into Beck's ass.