You could probably use some sort of buffer, poor a little down as a base then shot then more buffer to fill in the spaces and keep the shot from distorting the wadding. Is there any traditional buffers?
Yes you can use buffer, but no it does not help prevent distorting the wad. The purpose of buffer is to maintain the round shape of the shot. In modern shotguns, buffer can make a huge difference. In cylinder bore muzzleloaders, the jury is still out for me. I've never seen buffer hurt patterns though.
Now in my pressure testing modern shotguns, buffer raises pressures dramatically. I can only assume this would be true of a muzzleloader as well. Since our loads are usually very low pressure, it should not be an issue. I would not, however, take a 20 gauge, dump 2 ounces worth of buckshot down the tube on top of a hefty powder charge, and then buffer it. I wouldn't launch 2 ounces from a 20 gauge regardless, although people sometimes do it.
I really doubt there is a traditional buffer. In modern shotguns, most buffers involve plastic. Plenty of people use cream of wheat as buffer, and that would be more appropriate for a muzzleloader. Maybe grits would work too. You DO NOT want to use flour, or any other super fine and dense powder. It is extremely dangerous to do so.
The way to buffer a muzzleloading shotgun load is to pour your powder, and ram down your over powder wads. Then run a thin card wad down 2"-3". Next pour your shot in. At this point you can add buffer. It is a real PITA slow going with small birdshot, but it can be done. With buckshot, it is very easy. I like to add some buffer, then knock my knuckle on the side of the barrel to settle the buffer. Then repeat until the shot is fully filled and covered in buffer. You want the buffer between the shot, not mixed in with it. You want to knock on the barrel, such that the shot does not move, only the buffer does.
Now here is an important tip. Ram the shot down before you add an overshot card. I don't know exactly why, but if you add an overshot card first, it can be extremely difficult to seat onto the powder. This is not a method of loading you want to do all day, but if it tightens your buckshot pattern for deer, it is worth it. I like to swab the barrel between shots when loading this way, as it can be difficult to seat if the barrel is fouled.
One more buckshot tip. If you are using shot that is a tight fit, and not using buffer when stacking, say .310" balls in this .615" bore 20 gauge, I like to pour powder, then ram down all my over powder wads. Next I'll add shot by how many fit in a layer. In this example, 2 fit in a layer. I'll drop 2 down, then lightly tap them with my ramrod to make sure they are seated square. Don't jam it, just gentle 3" drop taps from the ramrod. Then repeat until I have as much shot as I want in there, and follow with an overshot card.
