Man you are trying to do too many things, at once, for me. You will have to unload your gun, reload with the slugs and draw a bead on a Buff which will have been charging, at you, for several seconds. Unless he was a decent distance away from you, I am putting my money on the Buff. No offense. Staying alive is your only choice and how you do it does not matter.
30 years ago I killed a charging Brown bear at less than 30 yards with a .270. One shot, in the left eye socket, and down he went. Two seconds more and the Brown bear would be typing here, if he learned how to type. I was lucky. Very under gunned, scope cranked up to max. power so I could not find a thing for the longest two seconds of my entire life, not ready for any charge, unaware that he was there, or that he found the other white meat, me, so tasty. And you are willing to smack a Buff, which I fear is several times harder to kill than a Brown bear, with a couple Breneke slugs.
You have to be either desperate or brave. Do you think that you will be able to stop a charging Buff, with one or two slugs, at ranges that you are dealing with? What is the chance of putting your slug in a key area, getting enough penetration to reach a vital area and then having enough time for it to work before the Buff is smoking a cigeratte over you body? No heart/lung shot will do, you either need a brain shot or a spinal cord shot. Rake him from mouth to anus and you are still dead. Before he bleeds out you will be long gone. Three legs will serve him long enough to finish you. I doubt that the slugs would break his leg for a certain fact. Brain or spine only as your target.
Now this is not sporting, and some will be repelled by the suggestion, but I would go for two shots to the eyes with the small pellets you already have in the gun. Not sporting, but I garuntee you have a better chance of putting out both of his eyes, than you have of putting out his lights or punching his ticket. Then you will have to return later and dispatch him with a proper solid load that will end his life.
Not fair chase, not a nice thing to do, but in the circumstances it would be one way to stay alive. Take out his eyes and you still have to deal with his nose and ears but at least you have some chance. Keep a large tree between the two of you and hope if he charges again he breaks his neck.
For that matter if you had to shoot a 3" shell in your gun it might not matter any way. There are four possible outcomes. If the gun blows up you are injured if not killed the Buff kills you, or the gun does not fail, you miss the instant fatal shot and the Buff kills you, the gun fails you are severly injured but do hit and kill the buff then die a short time later, or the gun holds togeather you hit and kill the Buff and live to tell us about it. Four outcomes of which three suck. 3:1 odds against a happy outcome.
Think that I would find another hobby unless you are willing to play unfair. Being fair is for the birds when it is your life at stake. No fighter pilot warned the other pilot if he could sneak in behind him and shoot him down first. Staying alive is the name of the game, let us not be PC and play by the fair chase rules and give a Buff an even break. The only even break I would like to give him is both front legs or his neck.
