It could be.
Here's a way to find out for sure, and if not, to clean it out.
This is for SMOOTH BORE GUNS ONLY....NEVER do this to a rifled bore.
Get some 0000 steel wool and wrap some around a bore brush.
Put the brush on a shotgun cleaning rod and chuck the rod in a drill.
Running the drill at medium speeds run the brush up and down the bore and chamber, keeping it moving constantly.
You can give it slightly more time on the spot area.
Polishing will take no more than a couple of minutes, so don't over do it.
After polishing, clean the bore with solvent and patches to remove the residue and inspect.
After inspection, apply a thin coat of lubricant in the bore to prevent rust.
This will polish the bore and clean out any stubborn fouling. If the spot is pitting, this can't remove it, but if the gun is worth it, you can buy a special barrel hone and special honing oil from Brownell's.
This tool can polish a bore until shallow pitting is polished out, but the tool is expensive and you MUST use the special oil or the hone will be destroyed.