Mossberg 500 wear or damage marks

[D53]

I picked up a used Mossberg 500 from a local shot the other day, checked the barrel and took a peak inside the action to make sure everything looked good. No issues. I took the shotgun out and shot about 200 rounds at clays with no hick ups. I went to clean the gun and when i took the bolt out, I noticed some gashes in the receiver. Honestly, I really am not too worried about them. I am more curious on what would have caused these markings in the receiver? The only thing I could come up with would be a very small lodged rock.
I circled the spots in red. The yellow circles from other marks, but those look more like just regular wear marks from use. Thanks for any info.

http://i388.photobucket.com/albums/oo321/cmb5383/Gun%20tech/M500.jpg (http://s388.photobucket.com/user/cmb5383/media/Gun%20tech/M500.jpg.html)

[Averageman]

I would say its pretty normal break in wear.
It would appear to me that those little circles were part of the machining process and they had a bit of excess metal over the top. As you fired and cycled the bolt just kept taking a bit off at a time until the internal coating and high spots on the metal have worn down.
What does the top of the bolt look like?

[Averageman]

I came home and checked mine tonight, it looks very similar except it is a bit more worn in the same area's

[D53]

Thanks for the reply. I double checked the top of the bolt when I got home from work today and there is little wear around the edges where the metal would make contact on the receiver. But nothing to indicate a broken or chipped piece of the bolt.

[Ron3]

It almost looked like someone was in there with a tool?

This reminds me of a relative's Mossberg 500 I need to fix. He got it used and when took it to the range only to find the slide "often" doesn't unlock the bolt after firing / dropping the hammer. In other words you have to use the bolt release button even after firing..sometimes.

Doesn't matter how simple the weapon they can have issues. Oh and of course this weapon had been his first line of defense for a couple months before realizing it had a problem. (he had never fired it)

[JBecker 72]

My 500 has a giant gouge in it from when a guy who was shooting mine short stroked it. When he did it somehow extracted but not ejected the spent hull, and attempted to chamber another round causing a double feed. He then proceeded for about 20 seconds to get the gun back into battery before I caught him and cleared the malfunction for him. The steel base on the hull had dug into the softer aluminum. I had to take a small file and knock the gouge down a bit to smooth the action back out. But it's been functioning just fine for a couple thousand rounds since.

Most likely the marks you are showing are just normal wear on the finish from hulls being chambered, extracted, and ejected. The finish on these seems to be pretty easy to damage. I wouldn't worry about it.

[Averageman]

Most likely the marks you are showing are just normal wear on the finish from hulls being chambered, extracted, and ejected. The finish on these seems to be pretty easy to damage. I wouldn't worry about it.
I'm pretty sure those holes are where during the machining process they are joining two pieces of metal with a spot weld. The resulting high points are left over material that wasn't cleaned up before it was assembled. The result is when cycling the bolt the excess is being worn down by metal on metal friction.
Wondering if someone with a NIB 500 could disassemble it and take some pictures just to validate that?