Just looking at the picture and not have the barrel in hand per say to look at it.....
I'll say the lighter/greyish colored area is carbon fouling.
The really black dark spot could be pitting in the steel? Was it from cleaning? Possibly.
It could be a void in the material as well but I've never really seen a void look like that at all. If it is a void look around that area of the barrel and see if you see anymore. If you do then I'll say a void in the material. How that happens is from impurities in the material. If you didn't see it when the barrel was new what happens is the voids where just below the surface. Then as you shoot the pressure for the bullet going down the bore etc...will make the surface sink into the voids. Kind a like a sink hole showing up in your yard.
Again though I've never seen voids with a greyish colored area around them. Even after being shot.
I'll say no to the ammonia cleaner causing it per say. I've used Sweet's in barrels since the late 80's and I've never had a problem with Sweet's. Never mix your cleaners together. That can cause chemical reactions and cause etching/pitting in the bore of a barrel. Unless your a chemist and you know how the stuff is going to react....don't do that. We've left Sweet's sit in a bore of a barrel for an extended period of time, put a piece of barrel and let it soak/submersed in the jar for months and nothing happened. If I recall correctly Boots (Obermeyer) did a similar test and didn't have a problem. Then he took the same hunk of barrel and coated it in Sweet's and let it sit on top of the hot water heater and then presto it pitted/corroded. Now did the heat and moisture and or some sort of electricity/current from the water heater cause it to react with the cleaner to cause the chemical reaction and cause the pitting. I'll say yes but like I'm saying...what and how it happened to one guy another guy might never see a problem due to all the circumstances involved.
You say the pit/problem is about a .5" in from the muzzle? It's not uncommon to see pitting at the muzzle end of the barrel more than anywhere else. Why? When the bullet leaves the barrel it creates a vacuum and sucks in the air from the outside into the bore of the barrel. Any moisture/junk in the air etc...that gets sucked in will deposit itself more in the very end of the muzzle area of the bore.
Also did you put the gun away dirty for an extended period of time with out cleaning it? If so the moisture in the air etc...will react with the fouling and pitting will occur. It's just a matter of time. The barrel steel is stainless steel but isn't surgical stainless. It will pit/it will corrode. This gets effected more or less with the type of cleaners (some cleaners are more water based then say oil based) that can be used, to the environment where you live/shoot (one guy who leaves near the ocean in a real humid environment etc...vs a guy who lives in Phoenix, AZ etc....) and or not cleaning it and putting it away dirty like I said.
Not to long ago we had a customer that put his rifle away for almost a year and didn't clean it. The face of the crown area was all pitted from the carbon build up. He got lucky as the bore didn't get effected by any pitting. The barrel did have a muzzle brake on it also.
If it's not effecting accuracy.....shoot the gun and just monitor it. If the accuracy is suffering and like has been suggested....cut the .5" off the end of the muzzle and recrown the barrel and shoot it.
Later, Frank
Bartlein Barrels
