I still think that it is difficult to beat the cost and efficacy of aluminum phosphate (phos-toxin, phos-tox, etc) tablets in gopher burrows. All these injection systems, whether we're talking about the gopher blaster, PERC, or other injection systems -- take too much time. With a phos-toxin tablet system, I can open a burrow and dump in three to six tablets quickly, and if the soil is too dry, I just water 'em down and move on to the next burrow.

This goes even moreso for squirrel systems -- I can just walk along, dropping phos-tox tablets into squirrel holes very rapidly, with water applied from a backpack sprayer. The phosgene gas resulting from these zinc or aluminum phosphate tablets is far more lethal than CO is.