Brake barrel airguns are not powerful enough to do the job. You can fire 45 long colts in a break barrel .410 shotgun all day long safely. .410s are 45-46 caliber. So, if you had a single shot .457 such as an AEA Element Max 457. Which I think is an awesome gun, I had one. You could get a barrel reamer from Brownell's and ream out the rifling or use an extended drill bit too cut it out then polish the bore. Then load it like a muzzle loading shotgun with a ram rod. It would do better with solid shot cup wads and fiber wads. They used to have aftermarket adjustable external chokes that would be easier to install than an internal choke using a die and some cutting oil to cut the threads. If there isn't enough diameter to cut the threads you could use a dowel as a jig to hold it straight and some JB weld on it. Here is some screw on 410 chokes. The biggest draw back with air shotguns is having enough pellets too have a good pattern at a fast enough velocity to make enough FPE per pellet to kill. In general, you have to go up from using #6 shot to using #4 shot instead to get enough FPE. I experimented quite a bit with a Seneca wing shot loading it with wads using a ram rod. The gun wasn't powerful enough in my opinion past about 15 yards with shot. But it was amazing shooting air bolts.