I'm not sure exactly what the term Shackle a Gun means, but I have an old H&A 16 gauge single which has killed a couple of snakes & remained unharmed. One was a rattler & the other a watersnake, not a cottonmouth. I killed it because he made me mad. Don't recall just how old I was but was dangling worms or grasshoppers or something from the door of an old Grist Mill which stood on our property not far from the house. Had caught a little bluegill & as I was pulling it up this snake slid out from under the building & stretched up trying to get my fish off the line. Pulled the fish on up out of reach & went to the house & got Dad's old 16. Went back out there & let the little'gill back down to the water & splashed it around a bit & here came that snake again. Pulled the fish up just out of reach & that old snake kept stretching up trying to get it with his mouth wide open so I put the charge right down into that gaping mouth from about 6-8 feet.
