Not sure if this has been posted before, but could not find if it has. What would be the minimum muzzle diameter for a .510 bore?
Thanks in advance.

Look at a 500 NE double rifle; it is really thin. Yes, this has has been discussed many times. .060 or so. Pressure at muzzles is low.
Search for it so I won't have to dig and measure one. But I will.

Must have been a typo.

Did you mean .60" diameter?
Even at .60, the .045" walls are less than 1/16" thickness.
Maybe I missed something.

Pretty sure he means wall thickness.

Correct; wall thickness of course. Because it applies to more than just a .510 groove barrel.

I don't disagree with Toms number, but wanted to add something that's important overall.

The bigger the caliber, the thicker the wall needs to be; because hoop strength needed varies directly as the projected area perpendicular to the hoop stress plane.

So, a .50 cal needs twice the barrel thickness of a .25 cal, for the same pressure.

The hoop stress per inch on a .510 barrel at 10,000 psi is .510x.510x10000 pounds or 2500 pounds. At 70,000 psi steel, the area needed would be 2500/70,000 = .0357 square inches per inch of barrel, for a thickness of .0357. So, Tom's .060" would have a factor of safety of nearly 2. That's presuming 10,000 psi at the muzzle, which is probably high.

Thank you all for the replies.

So, now that I know that dpcd did mean the wall thickness, the answer to the OP's question according to dpcd would .510 + .120 (.060 X 2) = .630" minimum muzzle diameter of 5/8 inches-ish. Correct?

Also the bore is .500" and grooves are .510.