I shoot a modern, cartridge slug bun in 12 ga. and it will put 5 shots in one hole at 50 yds, using iron sights, and standing off-hand, when I do my part. Now, I had to shoot 5 or 6 brands of shells to find the one that worked the best in that barrel, but now I am set.
The same should go with your 12 ga. ML. First, measure the bore, to be sure about its exact dimension. Then buy some balls in various diameters, and try different thicknesses of patches, and various lubes, to see what works best. I find that using an overpowder card behind these large round ball seals the bore from gas blow-by, saving both the patching and the round ball from being cut. Then its up to you and your skill as a shooter to make small groups. But most shooters can keep them in a 4 inch circle at 50 yds using a rest, and 8 inches shooting off-hand. That is good enough for deer hunting at those ranges. Remember you are shooting a projectile that is almost 3/4" in diameter, and it carries a tremendous wallop when it hits an animal. It creates a huge primary wound channel that bleeds massively immediately, and is too large to plug up or heal over easily. A deer hit in the chest is going to die fairly quickly.
The work you do in using better sights on your gun, for the light conditions you will be hunting in, and working up an accurate load on the range with your gun reflects your own hunting ethics. There are still guys out there who hunt with a shotgun that has no rear sight, with ammo they buy off the shelf and have never shot at paper to see where it hits, and who would and cripple deer every season before they get lucky to kill one. There are men out there who would not recognize a deer track if it was in neon lights, and make no effort to follow up deer they have shot, if they don't immediately see a Lot of Blood! They don't practice with thei guns between seasons, to keep what shooting skills they have up, nor do they clean their guns from year to year. ( Its only had 3 shots fired through it!) They don't oil or check the gun to make sure its functioning before taking it back out into the field, nor check all the screws to make sure something has not worked loose. They will use ammo that has been rolling around in the back of their pickup truck all year in all kinds of weather, and want to blame the manufacturer if it doesn't fire!
These are the kinds of men who, upon seeing me shoot my slug gun, want to know "Where can I buy a gun like that?" They don't notice the porting at the muzzle, and they can't see the long tapered forcing cone in the barrel that I had done to the factory barrel to make it into the accurate shooter it is. And, they would not understand the importance if they did see it. All they can understand is that the gun is capable of shooting very small groups at 50 yds.
