What size patched ball ...

.... in a open bore 12 gauge?

Measure the bore inside diameter with calipers. Take your patch material and multiply that by 2. Subtract the bore from the patch thickness. That is the general size of the ball you need. No need to tight patch, you are just removing the slop and centering the ball. You could even forget the patch and get a near bore size ball and put wadding under and over it.

Ball size is pretty much of a casual concern for smooth bored guns. @Dale Lilly will want to develop a load that will keep the ball more or less centered in the bore when shot. You don't need a tightly patched ball in a smoothbored gun as there are no grooves to take up the compression of patch material. So a ball that is smaller by 1.5 to 2 times the patch thickness is okay as you want some lubricant in the patching material to keep fouling soft, but enough hold to keep the ball in place while carrying and centered when shot. One can go a little tighter using a paper wrapped cartridge, but the 0.020" or more reduction in ball diameter will work as the escess paper that held the powder will act as a wad under the ball. The same relationship will work with a nest of tow fibers wrapped around the ball and a small ball of tow over the ball to hold it in place.

When shooting live rounds, I have used a ball about 0.050" (a 0.715 ball in a 0.770: bore) undersized in paper cartridges that I use in my Brown Bess. These work fine.

As I originally stated, ball size in a smooth bored gun is allowed a very wide tolerance as long as the ball is undersized to the bore. So much can be done with types of wads and patching materials. There is just no one specific answer for that individual gun.

As above smoothies are forgiving of what you feed them. I shoot a .690 in a .76 caliber and it shoots a small paper plate at fifty yards.
As the front sight is a bayonet lug i doubt I could get consistently well enough to get any smaller group
Shooting a bare .600 ball in my .62 I get good groups, but shooting a .575 in a paper military style cartridge I get deer hunting accuracy at fifty yards. Not as good as a bigger ball but not a shoot and hope
Being a twelve bore it should be around .72 a .690 would be my starting place, a .715 may work well without a patch but just a wad on top

Dale Lilly said:

.... in a open bore 12 gauge?

Click to expand...

Factory made shotties can, and do, often vary in bore size from what is stated. I have had a "12 ga" that actually was a 14 ga. Another supposedly 12 ga. that was actually an 11 ga. Smoothies can get dirty fast many people will use an undersized ball to be able to keep loading after several shots. Test and experiment to find what you like best is the only way to go in this game.

I used .690" balls in the 12 ga. I used to have. I have a 20 ga. now and use a .600" ball and sometimes a .590" ball.