Can they pass through indestructible surfaces? If fired at an adamantium wall, will they go right through it or will the wall stop the bullet? Can his bullets pierce indestructible skin (i.e. Superman) ? If fired at Mjolnir, will it go through Mjolnir or will Mjolnir stop the bullets? I am curious and I'd like to know. Thanks.

There is no way to know, since none of those things exist in the universe Preacher is set in. Any answer you get is pure speculation, and nothing more.

Hmmm....

Nope as it has never been tested or shown , or the bullets will drag the material along

Although they are not set in the same universe, SOK has made appearances in DC comics through the Hitman Series. So one could say that he has somewhat existed in the DC universe through myths. It could be possible that his bullets could kill Superman, since, well, they fucking killed The Devil and God. I'm not sure where his power ends. He seems to be an impenetrable omislayer. And for Mjolnir, I would have to say possibly not. It is unclear as to what it needed to destroy Mjolnir, even though it has been destroyed many times in the comics, but is done usually through dark magic, energy blasts from The Destroyer, and even himself pouring loads of energy into it. I think it's safe to assume that the bullets could potentially damage or even destroy Mjolnir, given the feats the gun's have. Conclusively, yes, I believe the bullets would in fact kill Superman or at-least damage or penetrate his skin. And Mjolnir would potentially get damaged or destroyed.